Specialty Topics · Emerging research topic

Microbiome Research and Mental Health

Q: Does gut bacteria cause depression?

Not shown. Studies find group differences, but most are case-control and affected by diet and medicines, and group differences cannot show the direction of cause.

Q: What is the gut-brain axis?

A 2019 review describes two-way communication between gut microbes and the brain through the immune system, tryptophan metabolism, the vagus nerve and the gut nervous system. Much of the detail comes from animals.

Q: Do probiotics help depression?

Some reviews report small to large symptom drops, but results vary a lot between reviews, trials are short and one analysis lost its effect without industry-funded trials. They are not an established treatment.

Q: Do probiotics help anxiety?

Effects were very small to moderate in two meta-analyses, and almost no trials were in diagnosed anxiety disorders.

Q: What are psychobiotics?

A term from 2013 for live organisms that, taken in adequate amounts, give a health benefit in people with psychiatric illness; widened in 2016 to include prebiotics.

Q: Does a Mediterranean-style diet help depression?

The SMILES trial and a five-trial meta-analysis found lower depression scores, but both are small or very varied and the meta-analysis rated certainty low. It is a supportive step, not a replacement for care.

Q: Do fermented foods help anxiety or stress?

Not shown. One four-week diet trial in healthy adults found no significant difference from control, and a survey cannot show cause.

Q: Is a fecal transplant a treatment for depression?

No. It is being tested. A 2026 trial added to an antidepressant did not show a different remission rate at week 8, and the FDA lists VOWST for preventing recurrent C. difficile infection.

Q: Can a gut microbiome test tell me if I am depressed or anxious?

No. No validated test was found, a commercial dataset showed no composition differences by screening score, and an expert panel says commercial tests lack proven clinical value.

Microbiome research and mental health (gut microbes, probiotics, prebiotics, diet, fecal transplants and stool tests studied for depression and anxiety) · also called Gut-brain axis research, psychobiotics, microbiome and depression, microbiome and anxiety, eje intestino-cerebro, microbioma y salud mental

Researchers ask whether the microbes in the gut influence mood and whether changing them could help depression and anxiety. Studies find differences in gut microbes between groups, but not that microbes cause these conditions. Probiotics show small to large symptom drops in some reviews with low certainty, prebiotics and fermented foods are unproven, fecal transplant is a medical procedure with one early depression trial, and consumer stool tests cannot diagnose anything. None of this is a replacement for mental health care.

General information, not medical advice. Last substantive update October 31, 2026 · Not medically reviewed. A named clinician has not signed off on this page. How this page is made

About Microbiome Research and Mental Health

Microbiome Research and Mental Health is a research topic, not a treatment. Studies of gut microbes find group differences in depression and anxiety, but not cause. Probiotics have small to large effects on symptom scores in some reviews with low certainty, prebiotics and fermented foods are unproven, fecal transplant is a medical procedure with one early depression trial, and consumer microbiome tests cannot diagnose these conditions. It is not a replacement for care.

What microbiome research and mental health means

Your gut is home to trillions of microbes. A 2019 review says they may communicate with the brain through the immune system, tryptophan metabolism, the vagus nerve and the nervous system of the gut, using chemicals such as short-chain fatty acids. Much of the work that built this idea was done in animals, and human studies are still catching up.

This page covers what studies of gut microbes, probiotics, "psychobiotics" and fecal transplants show about depression and anxiety, what they do not show, and why the research is not a replacement for care. It is about a research idea, not a treatment. It does not cover prescription medicines.

"Psychobiotic" was introduced in 2013 for a live organism that, taken in adequate amounts, gives a health benefit in people with psychiatric illness. In 2016 the term was widened to include prebiotics, and the authors noted that knowledge of how the microbiome responds to outside influence is still limited.

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Gut microbe differences in depression and anxiety: association, not cause

A 2021 review and meta-analysis of 59 case-control studies found a shared pattern across depression, bipolar disorder, schizophrenia and anxiety: fewer butyrate-producing bacteria (Faecalibacterium and Coprococcus) and more Eggerthella, with little sign that the pattern is specific to one condition. A large Flemish study of 1,054 people found Coprococcus and Dialister depleted in depression even after correcting for antidepressants.

The weak point is confounding. Reviews of anxiety and depression ask for better handling of diet and psychiatric medicines, which also change gut microbes. A 2025 review found conflicting results on which bacteria were more or less common, and 20 of its 24 studies were in China. Diversity findings were inconsistent.

A group difference also cannot tell you the direction. Microbes might affect mood, mood and its effects on sleep, diet and medicines might change microbes, or both. A systematic review of rodent studies that transferred microbes from ill people found 95% (36 of 38) reported disease features transferring, which the authors called implausibly high.

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Probiotics and "psychobiotics" for depression and anxiety: what trials show

A 2019 meta-analysis of 34 controlled trials found small effects of probiotics on depression (d = -0.24) and anxiety (d = -0.10), with larger effects in clinical samples. A 2025 meta-analysis of 23 trials in 1,401 diagnosed patients reported larger drops (depression SMD -0.96, anxiety SMD -0.59) with high heterogeneity. A 2023 review of seven trials in major depression found only modest effects and no consistent change in gut microbe diversity. A 2025 analysis of six trials in unmedicated adults found a small effect (SMD -0.38) that was no longer significant when industry-funded trials were excluded.

The reviews differ because they include different trials, products and people. Trials are short, mostly up to eight weeks, and strains, amounts and products vary. NCCIH says that in most instances we still do not know which probiotics are helpful and which are not. The result is low-certainty evidence that some products may modestly lower symptom scores in some people, not a recommendation.

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Prebiotics, Mediterranean-style diet and fermented foods

Prebiotics did not differ from placebo for depression or anxiety in the 2019 meta-analysis, and the 2025 analysis found no significant effect on depression.

In the SMILES trial, 67 adults with moderate to severe depression had either seven dietitian sessions or social support for 12 weeks. Depression scores fell more with diet support and remission was 32.3% versus 8.0%. A 2025 meta-analysis of five Mediterranean diet trials also found lower scores, but rated the evidence low certainty with very high heterogeneity.

A four-week diet high in fermented and prebiotic foods lowered perceived stress by 32% versus 17% in healthy adults, but the groups did not differ significantly. A survey of 710 young adults linked fermented food intake with less social anxiety in people high in neuroticism, which cannot show cause.

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Fecal microbiota transplant for depression: early research only

Fecal microbiota transplant (FMT) moves microbes from a healthy donor into a patient. Its established use is for recurrent Clostridioides difficile infection. The FDA page for VOWST lists the indication as preventing recurrence of that infection in adults, not depression.

In a 2026 randomized, double-blind trial in major depression, two weeks of FMT capsules or placebo were added to a standard antidepressant. Remission at week 8 did not differ significantly, although depression scores fell more with FMT at weeks 2 and 8. A 2025 pooled analysis of 12 trials in 681 people, many with irritable bowel syndrome, reported benefit that faded by 6 months. The evidence is small. Serious risks exist (see Safety), and FMT should never be a do-it-yourself project.

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Direct-to-consumer microbiome tests: what they cannot tell you

No validated stool test for diagnosing or screening for depression or anxiety was found. A commercial-test dataset from Japan found no significant composition differences among groups based on depression screening scores. A research model in 62 patients with bipolar depression and 60 healthy people reached an area under the curve of 0.733, but it was one study and no independent clinical validation was read.

An international expert panel says commercial microbiome diagnostic tests are offered without consensus on regulation or any proven value in clinical practice. When seven consumer services were tested with the same standard stool material they gave major discrepancies, as large as differences between donors. A test result cannot diagnose depression, anxiety or "gut-caused" mood problems.

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Safety first: not a replacement for care

Persistent low mood, anxiety, thoughts of self-harm or changes in sleep and appetite need assessment by a health professional. NIMH says treatment usually includes therapy, medication or both. NCCIH says not to use probiotics as a reason to postpone seeing your health care provider. Do not stop or change prescribed medicine for a probiotic, diet or test result without the prescriber.

Probiotics have a long record of apparently safe use in healthy people, but NCCIH says the risk of harm is greater in severe illness or weakened immunity, and bloodstream infections with probiotic organisms have been reported, mostly in children with central venous catheters. FMT carries risks too: two patients developed drug-resistant bloodstream infection from one donor and one died.

If you are thinking about harming yourself, contact local emergency services or a crisis line in your country, or go to the nearest emergency department. NIMH lists 911 and, in the US, 988 (call or text).

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At a glance

Traditional uses history, not proof

Studied for research

Probiotic supplements for depression, low certainty
Probiotic supplements for anxiety, very low certainty
Prebiotic fibres, insufficient
Mediterranean-style diet for depression, low certainty
Fermented foods and psychobiotic diets, insufficient
Fecal microbiota transplant, insufficient
Stool tests to diagnose depression, evidence against

Main compounds

Probiotics
Prebiotics
Diet pattern
Fermented foods
Fecal microbiota transplant
Stool microbiome tests

Common forms

Capsules and sachets of live organisms
Dietary programmes
Fermented foods
Clinic-delivered capsules, enemas or endoscopy (FMT)

Research links: Meta-analysis of prebiotic and probiotic trials for depression and anxiety (34 trials) · Meta-analysis of gut microbiota perturbations in psychiatric disorders (59 studies) · SMILES dietary improvement trial in major depression · Adjunctive FMT trial in major depression, 2026 · International consensus on microbiome testing in clinical practice · Research Library · Genes & Biomarkers Library

What it is and its history

Accepted scientific name: Microbiome research and mental health (a research topic, not a condition or a treatment)
Common names: Gut-brain axis research (English); Psychobiotics (English); Microbiota-gut-brain axis (English); Eje intestino-cerebro (Spanish); Microbioma y salud mental (Spanish)
Look-alikes and adulterants: Name: Probiotic supplements; Note: The products most often tested; see the probiotic supplements guide on this site.; Name: Depression and anxiety disorders; Note: The conditions studied; they have their own condition pages and established care described by NIMH and others.; Name: Irritable bowel syndrome; Note: A gut condition in which psychobiotics were first studied and which mixes into several FMT trials.

What is inside, and how it may work

What it contains and how those compounds act in cells and animals. Laboratory and animal mechanisms are not proven effects in people.

Main compounds

Compound	Family	Activity studied	Level of evidence
Probiotic supplements	live microorganisms	Studied as capsules or sachets, usually Lactobacillus and Bifidobacterium. Results vary widely by review. ^{14 1 17 11 7}	Human data
Prebiotic fibres	food for gut microbes	Did not differ from placebo for depression or anxiety in the meta-analyses read. ^{14 1}	Human data
Diet pattern (Mediterranean-style) and fermented foods	diet	A dietitian-led programme in the SMILES trial and a meta-analysis of five Mediterranean diet trials found lower depression scores; low certainty. A fermented-food diet did not beat control for stress in healthy adults. ^{10 3 2}	Human data
Fecal microbiota transplantation	microbial transfer (medical procedure)	One randomized trial in depression plus a mixed meta-analysis; approved by FDA only for preventing recurrent C. difficile infection (VOWST page). ^{13 26 31}	Human data
Direct-to-consumer stool tests	testing	Not validated for diagnosing depression or anxiety. ^{19 22 15}	Human data

Mechanisms, receptors and pathways

Target or pathway	What was found	Level
Communication routes between gut and brain	The 2019 review lists the immune system, tryptophan metabolism, the vagus nerve and the enteric nervous system, and microbial products such as short-chain fatty acids, branched chain amino acids and peptidoglycans. This is a description of proposed routes, largely from animal work. ²⁹	Animal and review
Loss of butyrate-producing, anti-inflammatory bacteria	The 2021 meta-analysis suggests these disorders show a reduction of anti-inflammatory butyrate-producing bacteria and enrichment of pro-inflammatory ones. This is a pattern in groups, not a shown cause. ^{18 24}	Observational
Psychobiotic action on the brain-gut axis	The 2013 paper proposes that some bacteria produce neuroactive substances such as GABA and serotonin and may act through the vagus nerve, spinal cord or neuroendocrine systems. At that time evidence came from rodents. ^{27 28}	Rodent (background)
Bile acids and inflammation (FMT trial)	In the 2026 trial the authors report a bile-acid-linked suppression of inflammatory pathways in a mediation analysis; this is a statistical link in one trial. ¹³	Human (mediation analysis)
Caution about causal claims from rodent transfer studies	A systematic review found 95% of human microbiota-associated rodent studies (36 of 38) reported transfer of disease features and called this implausibly high, saying it overstates the role of the microbiome in human disease. ³⁰	Systematic review of animal studies

Isolated compounds versus whole extracts

Probiotic supplements, prebiotic fibres, fermented foods and whole-diet changes are different interventions. NCCIH says different types of probiotics may have different effects, and what helps for one kind of Lactobacillus does not necessarily apply to another. Food-based changes act on many things at once, so a diet trial cannot tell which part worked.

7benefits and uses looked at

34studies and sources checked

Low certaintybest evidence rating found

6lab markers tracked in trials

2 low certainty1 very low certainty3 not enough evidence1 evidence against

What was looked at

The 7 benefits and uses checked for Microbiome Research and Mental Health, each with how strong the evidence is. Tap one to open it.

Probiotic supplements for depression symptomsLow certainty
Probiotic supplements for anxiety symptomsVery low certainty
Prebiotic fibres for depression and anxietyNot enough evidence
Dietary improvement (Mediterranean-style) for depressionLow certainty
Fermented foods and "psychobiotic" diets for stress, anxiety and moodNot enough evidence
Fecal microbiota transplant (FMT) for depressionNot enough evidence
Stool microbiome tests to diagnose or predict depressionEvidence against

Lab markers tracked in the studies

Gut microbe composition (stool microbiome)Depression symptom scoresAnxiety symptom scoresPerceived stress scoreRemission and HAMD-17 depression scoreAccuracy of microbiome-based classifiers (AUC)

What is Microbiome Research and Mental Health good for?

The main ways gut microbes have been studied for depression and anxiety, and how strong the human evidence is for each. These describe what studies found in the groups they studied. They are not advice to use any approach, they are not a replacement for mental health care, and each result applies to the product or programme tested.

Probiotic supplements for depression symptomsLow certaintySeveral reviews of trials found small to large drops in depression scores with probiotics compared with placebo, but the reviews disagree on size, the trials were short and varied, and one analysis lost its effect once industry-funded trials were removed. Probiotics are not an established treatment for depression. Probiotic supplements for anxiety symptomsVery low certaintyReviews found small to moderate drops in anxiety scores with probiotics, but fewer trials looked at anxiety than at depression, and for diagnosed anxiety disorders there is almost nothing. Probiotics are not an established treatment for anxiety. Prebiotic fibres for depression and anxietyNot enough evidencePrebiotics (food for gut bacteria) did not beat placebo on depression or anxiety in the two meta-analyses that tested them. Evidence is too thin to say they help. Dietary improvement (Mediterranean-style) for depressionLow certaintyOne small 12-week trial (SMILES) found that dietitian-led dietary support lowered depression scores more than social support, and a review of five trials found lower scores too, but the review rated the evidence low certainty and the trials differ a lot. Fermented foods and "psychobiotic" diets for stress, anxiety and moodNot enough evidenceA four-week diet high in fermented and prebiotic foods lowered perceived stress in healthy adults, but not more than the control diet. A survey linked fermented food intake with less social anxiety in people high in neuroticism. This is not enough to say fermented foods help mood. Fecal microbiota transplant (FMT) for depressionNot enough evidenceA 2026 trial of FMT capsules added to an antidepressant found no difference in remission at 8 weeks, although depression scores fell more in the transplant group, and a 2025 pooled analysis found benefit that faded by 6 months. FMT is not an approved treatment for depression and is a medical procedure. Stool microbiome tests to diagnose or predict depressionEvidence againstNo validated stool test for depression was found. A commercial-test dataset found no significant composition differences by depression screening score, and research models in bipolar depression have not been shown to work in independent groups.

Benefits, one by one

Each card is a research question, not a promise. Tap a goal to filter. Open a card to see the studies, doses and limits.

Probiotic supplements for depression symptoms

Low certainty

Several reviews of trials found small to large drops in depression scores with probiotics compared with placebo, but the reviews disagree on size, the trials were short and varied, and one analysis lost its effect once industry-funded trials were removed. Probiotics are not an established treatment for depression.

6 studiesThe trials used many different products, mostly Lactobacillus and Bif…

Studies, doses and limits

What the trials found

Reviews of controlled trials: 34 trials (2019 meta-analysis), 23 trials in 1,401 diagnosed patients (2025), 7 trials in major depressive disorder (2023), 13 trials in 437 adults (2026) and 6 trials in 341 unmedicated adults (2025).

Results

The 2019 meta-analysis found a small effect (d = -0.24), larger in clinical samples. The 2025 analysis of diagnosed patients found a large pooled drop (SMD -0.96) with high heterogeneity. The 2023 review found only modest effects. The 2026 review found most trials improved, mostly in mild to moderate outpatients with multi-strain products. In unmedicated adults the effect was small (SMD -0.38) and not significant once industry-funded trials were excluded.

Effect sizes as reported

d -0.24 (2019); SMD -0.96, CI -1.31 to -0.61 (2025); SMD -0.38, CI -0.57 to -0.18 (2025, unmedicated); SMD -0.21, CI -0.65 to 0.23 after excluding industry-funded trials.

Why this rating

Compiler's rating: low. The effect direction agrees across reviews, but size varies a lot, trials are short (mostly up to 8 weeks), strains and products differ, diet was rarely controlled, adverse events were poorly reported and part of the evidence is industry-funded.

Form and dose studied

The trials used many different products, mostly Lactobacillus and Bifidobacterium, often multi-strain, for about 4 to 8 weeks. This describes what was studied, not what anyone should take. NCCIH says that in most instances we still do not know which probiotics are helpful and which are not.

Who was studied

Adults with depression, from mild symptoms to major depressive disorder, mostly outpatients; some trials were in people also taking antidepressants.

Limits

Short trials, many strains, different countries, high heterogeneity, small samples, possible industry bias, no good long-term data.

Why researchers looked

The gut microbe differences seen in depression led researchers to test whether live bacteria in supplements could change mood.

Proposed mechanism Hypothesis

Proposed only: probiotics might act through the immune system, gut chemicals or the vagus nerve. The trials read measured symptoms, and one review found no consistent change in gut microbe diversity or composition after 4 to 8 weeks.

A proposed mechanism does not show that the effect happens in people.

Probiotics can cause serious infections in a few very ill people (see the Safety tab). They are not a reason to delay care or to change prescribed medicines.

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Probiotic supplements for anxiety symptoms

Very low certainty

Reviews found small to moderate drops in anxiety scores with probiotics, but fewer trials looked at anxiety than at depression, and for diagnosed anxiety disorders there is almost nothing. Probiotics are not an established treatment for anxiety.

3 studiesMany different products and strains, usually for several weeks; not a…

Studies, doses and limits

What the trials found

The 2019 meta-analysis of 34 trials and the 2025 meta-analysis of 23 trials in diagnosed patients (9 trials of probiotics for anxiety). A 2024 review of 42 studies in psychiatric diagnoses found only one study in generalized anxiety disorder.

Results

Probiotics had a very small effect on anxiety in the 2019 meta-analysis (d = -0.10, p = .03) and a moderate effect in the 2025 analysis (SMD -0.59) with high heterogeneity. The 2024 review said that, except in major depression, evidence does not clarify the role of probiotics in mental illness.

Effect sizes as reported

d -0.10 (2019); SMD -0.59, CI -0.98 to -0.19 (2025)

Why this rating

Compiler's rating: very low. The two meta-analyses differ about six-fold in size, anxiety trials are few and short, and diagnosed anxiety disorders were barely studied.

Form and dose studied

Many different products and strains, usually for several weeks; not advice on what to take.

Who was studied

Mixed groups: people with depression and anxiety symptoms, medical and community samples. Almost no data in diagnosed generalized anxiety disorder.

Limits

Few trials, wide size differences, short follow-up, high heterogeneity.

Why researchers looked

Anxiety often goes with depression, and animal work suggested gut bacteria influence stress responses, so trials added anxiety as an outcome.

Proposed mechanism Hypothesis

Proposed only, as for depression.

A proposed mechanism does not show that the effect happens in people.

Same cautions as for depression. Established care for anxiety disorders is on the anxiety page.

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Prebiotic fibres for depression and anxiety

Not enough evidence

Prebiotics (food for gut bacteria) did not beat placebo on depression or anxiety in the two meta-analyses that tested them. Evidence is too thin to say they help.

2 studiesNot described in the abstracts in plain units. The review called preb…

Studies, doses and limits

What the trials found

2019 meta-analysis of 34 trials (prebiotic arms) and a 2025 meta-analysis with 3 prebiotic trials in depression.

Results

Prebiotics did not differ from placebo for depression (d = -0.08, p = .51) or anxiety (d = 0.12, p = .11) in 2019. In 2025, depression SMD was -0.28 with a confidence interval that included no effect.

Effect sizes as reported

d -0.08 and 0.12 (2019); SMD -0.28, CI -0.61 to 0.04 (2025)

Why this rating

Compiler's rating: insufficient. Neither review found a significant effect, and only 3 prebiotic trials in depression were pooled in 2025.

Form and dose studied

Not described in the abstracts in plain units. The review called prebiotics food components that feed gut microbes.

Who was studied

Adults with depression or anxiety symptoms.

Limits

Very few trials; the 2025 authors still call for more research.

Why researchers looked

Prebiotics were tested as a way to feed the gut bacteria seen to be low in depression.

Proposed mechanism Hypothesis

Proposed only: feeding butyrate-producing bacteria. Not tested in these trials.

A proposed mechanism does not show that the effect happens in people.

Fibre-rich foods are a normal part of diet; added prebiotic powders can cause gas or bloating in some people (general knowledge, not from a source read).

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Dietary improvement (Mediterranean-style) for depression

Low certainty

One small 12-week trial (SMILES) found that dietitian-led dietary support lowered depression scores more than social support, and a review of five trials found lower scores too, but the review rated the evidence low certainty and the trials differ a lot.

2 studiesSMILES used seven sessions with a clinical dietitian over 12 weeks, a…

Studies, doses and limits

What the trials found

SMILES: 67 adults with moderate to severe depression for 12 weeks (diet 33, control 34). Meta-analysis: 5 randomized trials, 1,507 adults.

Results

In SMILES the MADRS fell more with diet support (Cohen's d -1.16), and remission was 32.3% versus 8.0%. The 2025 meta-analysis found lower depressive symptoms with Mediterranean diet interventions (SMD -0.53) but very high heterogeneity (87.1%) and a prediction interval that includes no effect.

Effect sizes as reported

SMILES d -1.16, remission 10 of 31 vs 2 of 25; meta-analysis SMD -0.53, CI -0.90 to -0.16.

Why this rating

Compiler's rating: low, matching the review authors' own GRADE rating. One small single-blind trial and a heterogeneous set of five trials; most participants in SMILES also had other treatment.

Form and dose studied

SMILES used seven sessions with a clinical dietitian over 12 weeks, as an add-on to other care. Its protocol paper is titled as a modified Mediterranean dietary intervention (title only read). This describes the trial, not a prescription.

Who was studied

Adults with moderate to severe depression (SMILES); adults aged about 22 to 53 with depression in the meta-analysis.

Limits

Small samples, single blind, very high heterogeneity, only one trial with remission data read, no long-term follow-up read.

Why researchers looked

Diet affects gut microbes and general health, and people with depression often eat poorly, so researchers tested diet change as a supportive step.

Proposed mechanism Hypothesis

Proposed only: diet might act through gut microbes, inflammation or general health. The trials read did not test these routes.

A proposed mechanism does not show that the effect happens in people.

Diet change is a supportive step, not a replacement for therapy or medicines. People with food allergies, eating disorders or medical diets should plan changes with a clinician.

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Fermented foods and "psychobiotic" diets for stress, anxiety and mood

Not enough evidence

A four-week diet high in fermented and prebiotic foods lowered perceived stress in healthy adults, but not more than the control diet. A survey linked fermented food intake with less social anxiety in people high in neuroticism. This is not enough to say fermented foods help mood.

4 studiesThe trial diet was high in prebiotic and fermented foods for 4 weeks;…

Studies, doses and limits

What the trials found

One randomized trial in 45 healthy adults (4 weeks), one survey of 710 young adults, and a 2024 review of 42 clinical studies that included fermented foods in its search.

Results

In the trial perceived stress fell by 32% versus 17%, but the groups did not differ significantly; higher adherence went with bigger drops. In the survey, fermented food intake linked to fewer social anxiety symptoms only for people high in neuroticism. The 2024 review could not clarify a role outside depression.

Effect sizes as reported

Trial: 32% vs 17% reduction, not significant between groups. Survey: interaction effect, no number in the abstract.

Why this rating

Compiler's rating: insufficient. One small trial in healthy people that missed its between-group comparison, plus one survey that cannot show cause.

Form and dose studied

The trial diet was high in prebiotic and fermented foods for 4 weeks; the survey measured how often people ate fermented foods. These describe what was studied.

Who was studied

Healthy adults (trial); young adults (survey).

Limits

Small, short, healthy participants, self-report, cannot show cause.

Why researchers looked

Fermented foods and fibre feed or add gut microbes, so researchers asked whether eating them could be a food-based form of a probiotic.

Proposed mechanism Hypothesis

Proposed only: more diverse microbes and their by-products. In the trial, microbial composition changed only subtly.

A proposed mechanism does not show that the effect happens in people.

Fermented foods are ordinary foods for most people, but some (such as very salty or aged products) do not suit everyone; people with weakened immune systems should ask their clinician (general guidance; see Safety).

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Fecal microbiota transplant (FMT) for depression

Not enough evidence

A 2026 trial of FMT capsules added to an antidepressant found no difference in remission at 8 weeks, although depression scores fell more in the transplant group, and a 2025 pooled analysis found benefit that faded by 6 months. FMT is not an approved treatment for depression and is a medical procedure.

5 studiesThe trial used FMT capsules for 2 weeks. Meta-analysed trials used ca…

Studies, doses and limits

What the trials found

One randomized, double-blind trial in major depression; a 2025 meta-analysis of 12 randomized trials (681 participants, many with irritable bowel syndrome); a 2020 systematic review that mostly covers animal work.

Results

In the 2026 trial remission at week 8 did not differ, but HAMD-17 scores fell more at weeks 2 and 8. The 2025 meta-analysis found a large pooled reduction (SMD -1.21) that fell to -0.56 in a sensitivity analysis and diminished by 6 months; the effect was smaller in people with neurological or psychiatric conditions (SMD -0.67).

Effect sizes as reported

SMD -1.21, CI -1.87 to -0.55; sensitivity SMD -0.56, CI -0.86 to -0.26; trial remission not significant.

Why this rating

Compiler's rating: insufficient. Only one randomized depression trial was read and its main outcome was null; the pooled analysis mixes in gut disease and shows fading effects. Evidence is small.

Form and dose studied

The trial used FMT capsules for 2 weeks. Meta-analysed trials used capsules, endoscopy or enemas. FMT is given by clinicians in controlled settings only; this is not advice.

Who was studied

Adults with major depression already taking a standard antidepressant (2026 trial); mixed gut and neuropsychiatric patients (2025).

Limits

One main trial; mixed populations; short follow-up; effects fading by 6 months.

Why researchers looked

Because FMT works for a specific bowel infection, researchers asked whether moving microbes from a healthy donor could also ease mood symptoms.

Proposed mechanism Hypothesis

Proposed: changes in bile acids and inflammation (the trial authors' mediation analysis). This is a statistical link in one trial, not proof.

A proposed mechanism does not show that the effect happens in people.

FMT carries real infection risks and is not available for depression outside trials (see Safety). Do not attempt do-it-yourself transplants.

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Stool microbiome tests to diagnose or predict depression

Evidence against

No validated stool test for depression was found. A commercial-test dataset found no significant composition differences by depression screening score, and research models in bipolar depression have not been shown to work in independent groups.

4 studiesStool samples sequenced in the lab; in research, random forest models…

Studies, doses and limits

What the trials found

Commercial-test dataset analysis in Japan; a case-control study with machine learning in 62 patients and 60 controls; an international expert consensus on microbiome testing (2025).

Results

No significant differences in composition by depression screening score in the commercial dataset. The research model reached an AUC of 0.733 for patients versus controls in a single study. The expert consensus says evidence supporting clinical usefulness of microbiome diagnostics is scarce and that commercial tests lack proven value in clinical practice.

Effect sizes as reported

AUC 0.733 and 0.800 in one research study; no significant differences in the commercial dataset.

Why this rating

Compiler's rating: against. The only independent-looking data found showed no difference, the accurate-looking model was a single-site research study, and an international expert panel says clinical value is unproven.

Form and dose studied

Stool samples sequenced in the lab; in research, random forest models were built from bacterial species.

Who was studied

Adults in Japan using commercial tests; patients with bipolar depression in China (research).

Limits

Small samples, no independent validation read, no clinical diagnoses in the commercial dataset.

Why researchers looked

If microbes differ in depression, perhaps a stool test could flag it or predict who will respond to treatment.

Proposed mechanism Hypothesis

A diagnostic idea rather than a treatment, so no mechanism is proposed.

A proposed mechanism does not show that the effect happens in people.

A test result cannot tell you whether you have depression. See a clinician for assessment.

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Connections to the rest of health

How Microbiome Research and Mental Health connects to the rest of our health knowledge graph. Every link says what kind of connection it is. A safety link (for example to the liver) is a warning, not a benefit. A study that merely mentions a topic never creates a benefit link.

Explore every connection: goals, body systems, organs, symptoms, genes

Kind of connection Connected to Search

A. Wellness goals

Connected to	Evidence status	Context	Studies
Mood Balance	Low certainty	Probiotic supplements for depression symptoms: Several reviews of trials found small to large drops in depression scores with probiotics compared with placebo, but the reviews disagree on size, the trials were short and varied, and one analysis lost its effect once industry-funded trials were removed. Probiotics are not an established treatment for depression.	^{14 1 17 11 7 32}
Anxiety Support	Very low certainty	Probiotic supplements for anxiety symptoms: Reviews found small to moderate drops in anxiety scores with probiotics, but fewer trials looked at anxiety than at depression, and for diagnosed anxiety disorders there is almost nothing. Probiotics are not an established treatment for anxiety.	^{14 1 21}
Mood Balance	Not enough evidence	Prebiotic fibres for depression and anxiety: Prebiotics (food for gut bacteria) did not beat placebo on depression or anxiety in the two meta-analyses that tested them. Evidence is too thin to say they help. Fermented foods and "psychobiotic" diets for stress, anxiety and mood: A four-week diet high in fermented and prebiotic foods lowered perceived stress in healthy adults, but not more than the control diet. A survey linked fermented food intake with less social anxiety in people high in neuroticism. This is not enough to say fermented foods help mood.	^{14 1 2 9 21 32 15 12 19 22}
Mood Balance	Low certainty	Dietary improvement (Mediterranean-style) for depression: One small 12-week trial (SMILES) found that dietitian-led dietary support lowered depression scores more than social support, and a review of five trials found lower scores too, but the review rated the evidence low certainty and the trials differ a lot. Fecal microbiota transplant (FMT) for depression: A 2026 trial of FMT capsules added to an antidepressant found no difference in remission at 8 weeks, although depression scores fell more in the transplant group, and a 2025 pooled analysis found benefit that faded by 6 months. FMT is not an approved treatment for depression and is a medical procedure.	^{10 3 13 26 5 6 31}
Brain Health	Low certainty	Dietary improvement (Mediterranean-style) for depression: One small 12-week trial (SMILES) found that dietitian-led dietary support lowered depression scores more than social support, and a review of five trials found lower scores too, but the review rated the evidence low certainty and the trials differ a lot.	^{10 3}
Gut Health	Not enough evidence	Fermented foods and "psychobiotic" diets for stress, anxiety and mood: A four-week diet high in fermented and prebiotic foods lowered perceived stress in healthy adults, but not more than the control diet. A survey linked fermented food intake with less social anxiety in people high in neuroticism. This is not enough to say fermented foods help mood.	^{2 9 21 32}

B. Body systems

Connected to	Kind of connection	Evidence status	Context	Studies
Nervous System	Biological association	Low certainty	Studied for: Probiotic supplements for depression symptoms Studied for: Probiotic supplements for anxiety symptoms	^{14 1 17 11 7 32 21 10 3 13 26 5 6 31}
Stress Mood	Biological association	Low certainty	Studied for: Probiotic supplements for depression symptoms Studied for: Probiotic supplements for anxiety symptoms	^{14 1 17 11 7 32 21 2 9}
Digestive System	Biological association	Not enough evidence	Studied for: Prebiotic fibres for depression and anxiety Studied for: Fermented foods and "psychobiotic" diets for stress, anxiety and mood	^{14 1 2 9 21 32 13 26 5 6 31 15 12 19 22}
Whole Body	Biological association	Low certainty	Studied for: Dietary improvement (Mediterranean-style) for depression	^{10 3}

E. Health conditions

Connected to	Evidence status	Context	Studies
Depression	Systematic reviews and trials, low certainty	Small to large symptom drops in some reviews; not established treatments. Preparation or compound: Probiotics, diet, fecal microbiota transplant	^{14 1}
Anxiety disorders	Systematic reviews, very low certainty	Very small to moderate effects on anxiety scores; almost no data in diagnosed disorders. Preparation or compound: Probiotics	^{14 1 21}
Recurrent C. difficile infection	Randomized trials and regulator approval	The setting where FMT has evidence and approval; not depression. Preparation or compound: Fecal microbiota-based products	^{16 31}

I. Biomarkers

Connected to	Kind of connection	Evidence status	Context	Studies
Gut microbe composition (stool microbiome)	Biological association	Mixed findings	Measured in trials: Group differences, not consistent: often fewer butyrate-producing bacteria (Faecalibacterium, Coprococcus) and more of some pro-inflammatory groups (such as Eggerthella); diversity results conflicted.. A lab change is not the same as a health benefit.	^{18 24 23 4}
Depression symptom scores	Biological association	Low certainty	Measured in trials: Lower scores with probiotics and diet support in several trials, with large differences between reviews.. A lab change is not the same as a health benefit.	^{14 1 7 10}
Anxiety symptom scores	Biological association	Very low certainty	Measured in trials: Small to moderate reductions with probiotics in two meta-analyses.. A lab change is not the same as a health benefit.	^{14 1}
Perceived stress score	Biological association	Not enough evidence	Measured in trials: Fell in both diet groups; the psychobiotic diet did not differ significantly from control.. A lab change is not the same as a health benefit.	²
Remission and HAMD-17 depression score	Biological association	Not enough evidence	Measured in trials: No significant remission difference at week 8; greater HAMD-17 reduction at weeks 2 and 8 with FMT.. A lab change is not the same as a health benefit.	¹³
Accuracy of microbiome-based classifiers (AUC)	Biological association	Very low certainty	Measured in trials: Moderate accuracy in one research study; not tested in independent groups in the sources read.. A lab change is not the same as a health benefit.	¹²

L. Active compounds

Connected to	Kind of connection	Evidence status	Context	Studies
Probiotic supplements	Biological association	Human	Studied as capsules or sachets, usually Lactobacillus and Bifidobacterium. Results vary widely by review.	^{14 1 17 11 7}
Prebiotic fibres	Biological association	Human	Did not differ from placebo for depression or anxiety in the meta-analyses read.	^{14 1}
Diet pattern (Mediterranean-style) and fermented foods	Biological association	Human	A dietitian-led programme in the SMILES trial and a meta-analysis of five Mediterranean diet trials found lower depression scores; low certainty. A fermented-food diet did not beat control for stress in healthy adults.	^{10 3 2}
Fecal microbiota transplantation	Biological association	Human	One randomized trial in depression plus a mixed meta-analysis; approved by FDA only for preventing recurrent C. difficile infection (VOWST page).	^{13 26 31}
Direct-to-consumer stool tests	Biological association	Human	Not validated for diagnosing depression or anxiety.	^{19 22 15}

Each connection is stored with the ingredient, the connected record, the kind of connection, the evidence status, the supporting studies, the preparation or compound involved, and a review date (last reviewed October 31, 2026).

Evidence at a glance: what the research found for 7 outcomes

2Low certainty

1Very low certainty

3Not enough evidence

1Evidence against

Each outcome below is one thing researchers measured. Certainty is how far the evidence can be trusted, not how well it works. The full studies are on the next tab.

Probiotic supplements for depression symptoms 6 studies Low certainty Probiotic supplements for anxiety symptoms 3 studies Very low certainty Prebiotic fibres for depression and anxiety 2 studies Not enough evidence Dietary improvement (Mediterranean-style) for depression 2 studies Low certainty Fermented foods and "psychobiotic" diets for stress, anxiety and mood 4 studies Not enough evidence Fecal microbiota transplant (FMT) for depression 5 studies Not enough evidence Stool microbiome tests to diagnose or predict depression 4 studies Evidence against

The evidence

The research centre for Microbiome Research and Mental Health. Every record here was checked against PubMed or the publisher's page; we never list a study we have not verified.

Evidence by outcome

Bars show how many studies we found for each outcome, not how well it works. People are added up across trials only (reviews are not added, to avoid double counting).

How we rate evidence

High certainty

Several well-run trials or reviews agree; further research is unlikely to change the picture much.

Moderate certainty

Good trials point the same way, but some doubt remains.

Low certainty

Small, short or inconsistent trials. The finding could change with better research.

Very low certainty

Very little or very weak human data. Treat as a possibility only.

Mixed findings

Trials or reviews disagree with each other.

Not enough evidence

Not enough human research to say either way.

Evidence against

Good research found no benefit, or found harm.

We take the reviewers' own risk-of-bias and GRADE statements where they exist, and otherwise judge from study size, design, consistency, directness and whether a product matches what is sold. Many publications do not make evidence high. Method note: We searched PubMed, read the abstract of every review, trial and study cited, and read the NCCIH, FDA and NIMH pages as downloaded text. Full texts were not read. Certainty ratings are the compiler's own reading unless a review gives its own grade (stated in the text). Funding is not stated in most abstracts, and this is noted where relevant.

Study library: every study we checked 34

Design Research type Year from Search

Study	Design	People	Product, dose, length	Result
Asad 2025 Effects of Prebiotics and Probiotics on Symptoms of Depression and Anxiety in Clinically Diagnosed Samples: Systematic Review and Meta-analysis of Randomized Controlled Trials. Nutr Rev · PMID 39731509 · DOI	Systematic review and meta analysis of randomized trials	23 randomized trials, 1,401 patients with clinically diagnosed depression or anxiety symptoms (20 trials in the meta-analysis)	Probiotics, prebiotics or synbiotics Mostly up to 8 weeks · vs Placebo	Probiotics lowered depression symptoms (18 trials) and anxiety symptoms (9 trials) more than placebo. Prebiotics (3 trials in depression) did not show a significant effect. Heterogeneity between studies was high; study length and formulation contributed to the differences. Probiotics: depression SMD -0.96, anxiety SMD -0.59. Prebiotics: depression SMD -0.28. (Depression -1.31 to -0.61; anxiety -0.98 to -0.19; prebiotics -0.61 to 0.04) Methods, limits and funding Outcomes: Depression and anxiety symptom scores Limitations: High heterogeneity; short trials; many different products. Pooled effects this large are unusual for this field and should be read with caution. Funding and conflicts: Funding is not stated in the abstract. Registration: PROSPERO CRD42023424136 Read: Abstract read through PubMed (esummary and efetch) on 2026-10-31; full text not read.
Berding 2023 Feed your microbes to deal with stress: a psychobiotic diet impacts microbial stability and perceived stress in a healthy adult population. Mol Psychiatry · PMID 36289300 · DOI	Randomized controlled trial (diet)	45 45 healthy adults (24 psychobiotic diet, 21 control diet)	Psychobiotic diet (high in prebiotic and fermented foods) 4 weeks · vs Control diet	Perceived stress fell 32% on the psychobiotic diet and 17% on the control diet, but the difference between groups was not significant. Stress biomarkers were not affected. Microbial composition changed only subtly. Higher adherence to the diet went with bigger drops in perceived stress. Perceived stress 32% vs 17% reduction; between-group difference not significant (Not given in the abstract) Methods, limits and funding Outcomes: Perceived stress; stool microbiome; metabolites Limitations: Small, short trial in healthy adults, so it says little about people with depression or anxiety. Funding and conflicts: Funding is not stated in the abstract. Read: Abstract read through PubMed (esummary and efetch) on 2026-10-31; full text not read.
Bizzozero-Peroni 2025 The impact of the Mediterranean diet on alleviating depressive symptoms in adults: a systematic review and meta-analysis of randomized controlled trials. Nutr Rev · PMID 38219230 · DOI	Systematic review and meta analysis of randomized trials	1507 5 randomized trials, 1,507 young and middle-aged adults with major depression or mild to moderate depressive symptoms	Mediterranean diet interventions Varied · vs Control conditions	Mediterranean diet interventions reduced depressive symptoms compared with control conditions. Overall risk of bias ranged from some concerns to high and the certainty of evidence was low. SMD -0.53; I2 87.1%; prediction interval -1.86 to 0.81 (-0.90 to -0.16) Methods, limits and funding Outcomes: Depressive symptom severity Limitations: Very high heterogeneity, only 5 trials, low certainty by the authors' own GRADE rating. The prediction interval includes no effect. Funding and conflicts: Funding is not stated in the abstract. Registration: PROSPERO CRD42022341895 Read: Abstract read through PubMed (esummary and efetch) on 2026-10-31; full text not read.
Cao 2025 Gut microbiota variations in depression and anxiety: a systematic review. BMC Psychiatry · PMID 40312666 · DOI	Systematic review of case control studies	24 articles; 20 of them conducted in China	Gut microbiota comparisons Cross-sectional · vs People without depression or anxiety	Diversity findings were inconsistent for both depression and anxiety. Both were characterised by more pro-inflammatory bacteria and fewer anti-inflammatory short-chain fatty acid producers, but reports conflicted because of confounders such as diet and psychotropic medicines. Not given (Not given) Methods, limits and funding Outcomes: Alpha and beta diversity; taxa abundance Limitations: Mostly one country; conflicting reports; qualitative synthesis only. Funding and conflicts: Funding is not stated in the abstract. Read: Abstract read through PubMed (esummary and efetch) on 2026-10-31; full text not read.
Chinna Meyyappan 2020 Effect of fecal microbiota transplant on symptoms of psychiatric disorders: a systematic review. BMC Psychiatry · PMID 32539741 · DOI	Systematic review human and animal	21 studies of fecal microbiota transplantation in relation to psychiatric symptoms; the abstract describes clinical, human-donor preclinical and entirely preclinical studies	Fecal microbiota transplantation Varied · vs Varied	The abstract says all studies found a decrease in depressive and anxiety-like symptoms after transplantation of healthy microbiota, and that depressive and anxiety-like behaviours were transmitted from ill donors to healthy recipients. It concludes the evidence is strong but calls for larger, better-designed studies to establish efficacy and safety. Not pooled (Not pooled) Methods, limits and funding Outcomes: Mood and behaviour measures Limitations: Most of the evidence is from animals; the counts in the abstract (8 clinical, 9 preclinical with human donors, 11 preclinical) do not add up to 21, so only the existence of a small clinical group is relied on here. The claim of "strong evidence" is not supported by the controlled human data. Funding and conflicts: Funding is not stated in the abstract. Registration: PROSPERO CRD42019126795 Read: Abstract read through PubMed (esummary and efetch) on 2026-10-31; full text not read.
DeFilipp 2019 Drug-Resistant E. coli Bacteremia Transmitted by Fecal Microbiota Transplant. N Engl J Med · PMID 31665575 · DOI	Case report (two patients in two trials)	2 Two patients who developed ESBL-producing E. coli bloodstream infection after fecal microbiota transplantation in two independent clinical trials	Fecal microbiota transplantation No comparison group · vs No comparator	Both infections were linked to the same stool donor by genomic sequencing. One of the patients died. The authors call for enhanced donor screening and continued vigilance about benefits and risks in different patient groups. No effect size reported (Not given) Methods, limits and funding Outcomes: Infection linked to donor stool Limitations: Two cases; shows that serious harm can occur, not how often. Funding and conflicts: Funding is not stated in the abstract. Read: Abstract read through PubMed (esummary and efetch) on 2026-10-31; full text not read.
Haiyan 2025 Efficacy of probiotic intervention in unmedicated depression: a systematic review and meta-analysis. Front Psychiatry · PMID 41573044 · DOI	Systematic review and meta analysis of randomized trials	341 6 randomized trials of probiotic treatment in adults with depression who were not taking psychotropic medicines (169 probiotic, 172 placebo)	Probiotic supplements Not given in the abstract · vs Placebo	Probiotics were linked to a small reduction in depressive symptoms. When industry-funded trials or trials with add-on agents were excluded, the result was no longer significant. Only minor adverse events were reported, with no difference between groups and no serious events. SMD -0.38; after excluding industry-funded trials or add-on agents SMD -0.21 (-0.57 to -0.18; sensitivity analysis -0.65 to 0.23) Methods, limits and funding Outcomes: Change in validated depression scales; safety Limitations: Small trial set; possible industry bias; heterogeneity (I2 51%). Funding and conflicts: The abstract reports a sensitivity analysis by funding source, which shows part of the trials were industry-funded. Registration: PROSPERO CRD420251015474 Read: Abstract read through PubMed (esummary and efetch) on 2026-10-31; full text not read.
Hefter 2023 An 11-Year Review of Lactobacillus Bacteremia at a Pediatric Tertiary Care Center. Hosp Pediatr · PMID 37203378 · DOI	Retrospective case review	127845 Lactobacillus bacteremia over 11 years at a pediatric hospital (127,845 admissions)	Hospital probiotic use 11 years · vs No comparison group	Eight cases were found. All had signs of infection. Most had impaired intestinal function and a central venous catheter. Three had a history of probiotic use. The authors call it uncommon but say certain populations may be at higher risk. 8 cases per 127,845 admissions (Not given) Methods, limits and funding Outcomes: Cases of Lactobacillus bloodstream infection and their risk factors Limitations: Single hospital; children; small number of cases. Funding and conflicts: Funding is not stated in the abstract. Read: Abstract read through PubMed (esummary and efetch) on 2026-10-31; full text not read.
Hilimire 2015 Fermented foods, neuroticism, and social anxiety: An interaction model. Psychiatry Res · PMID 25998000 · DOI	Cross sectional survey	710 710 young adults (445 female) who completed questionnaires	Self-reported frequency of eating fermented foods One time point (survey) · vs Lower frequency of fermented food intake	Fermented food intake, neuroticism and exercise each predicted social anxiety. For people high in neuroticism, more frequent fermented food intake went with fewer social anxiety symptoms. The authors say the direction of cause is unknown. Interaction between fermented food intake and neuroticism (Not given in the abstract) Methods, limits and funding Outcomes: Social anxiety symptoms Limitations: Self-report survey at one time; cannot show that fermented foods reduced anxiety; other habits may differ between people. Funding and conflicts: Funding is not stated in the abstract. Read: Abstract read through PubMed (esummary and efetch) on 2026-10-31; full text not read.
Jacka 2017 A randomised controlled trial of dietary improvement for adults with major depression (the 'SMILES' trial). BMC Med · PMID 28137247 · DOI	Randomised controlled trial (single blind)	67 67 adults with moderate to severe depression (33 diet, 34 control); 55 were using some form of therapy; 56 had complete 12-week data	Seven individual nutritional counselling sessions with a clinical dietitian (dietary improvement programme) 12 weeks · vs Social support sessions on the same schedule	The diet group improved more on the MADRS than the social support group. Remission (MADRS below 10) was reached by 32.3% (10 people) of the diet group and 8.0% (2 people) of the control group. Cohen's d -1.16; number needed to treat 4.1 (NNT 95% CI 2.3 to 27.8) Methods, limits and funding Outcomes: Montgomery-Asberg Depression Rating Scale (MADRS) at 12 weeks; remission Limitations: Small, single-site, single-blind trial; participants knew which sessions they attended; the very wide NNT interval shows the uncertainty. A large effect from a small trial needs repeating. Funding and conflicts: Funding is not stated in the abstract. Registration: ACTRN12612000251820 Read: Abstract read through PubMed (esummary and efetch) on 2026-10-31; full text not read.
Johnson 2026 The therapeutic readiness of probiotics in major depressive disorder: A systematic review of randomised controlled trials. J Psychopharmacol · PMID 41453849 · DOI	Systematic review of randomised trials	13 double-blind, placebo-controlled randomised trials, 437 adults aged 18 to 75 with major depressive disorder	Probiotics (many were multi-strain Lactobacillus and Bifidobacterium products) Mostly 4 to 8 weeks · vs Placebo	Most trials reported improvements in depressive symptoms, particularly in outpatients with mild to moderate depression using multi-strain products. Benefit was most consistent in Iranian outpatients, more modest in European inpatients and variable in East Asian outpatients. The authors describe probiotics as appearing safe and well tolerated, with modest adjunctive benefit. Not pooled (Not pooled) Methods, limits and funding Outcomes: Depressive symptoms and secondary outcomes Limitations: Attrition, selective reporting and method differences remained sources of bias; results varied by country. Funding and conflicts: Funding is not stated in the abstract. Read: Abstract read through PubMed (esummary and efetch) on 2026-10-31; full text not read.
Lai 2022 Metagenomic analysis reveals gut bacterial signatures for diagnosis and treatment outcome prediction in bipolar depression. Psychiatry Res · PMID 34896845 · DOI	Case control study with machine learning	122 62 patients in a depressive episode of bipolar disorder and 60 healthy people	Stool metagenomic sequencing; random forest models Patients were treated for one month with a prescription medicine · vs Healthy individuals; responders vs non-responders	Patients showed specific alterations in gut microbial diversity and composition. Models using bacterial species separated patients from controls with an area under the curve of 0.733 and responders from non-responders with 0.800. AUC 0.733 (patients vs controls); 0.800 (responders vs non-responders) (Not given in the abstract) Methods, limits and funding Outcomes: Microbial differences; model accuracy Limitations: Single-site, small, bipolar depression only; the abstract does not describe testing in an independent group; treatment itself changed the microbiome. Funding and conflicts: Funding is not stated in the abstract. Read: Abstract read through PubMed (esummary and efetch) on 2026-10-31; full text not read.
Li 2026 Adjunctive fecal microbiota transplantation for major depressive disorder: A randomized, double-blind, placebo-controlled trial. Cell Host Microbe · PMID 42309058 · DOI	Randomized, double blind, placebo controlled trial	Patients with major depressive disorder (number not given in the abstract) who were also receiving a standard antidepressant	Fecal microbiota transplantation capsules 2-week course; outcomes at weeks 2 and 8 · vs Placebo capsules	Remission at week 8 did not differ significantly between groups. HAMD-17 scores fell more in the transplant group at weeks 2 and 8. Safety was comparable to placebo. The authors report donor microbes persisting and a link with bile acid changes. Remission no significant difference; HAMD-17 greater reduction at weeks 2 and 8 (Not given in the abstract) Methods, limits and funding Outcomes: Remission rate at week 8; HAMD-17 scores; safety; microbial engraftment Limitations: One trial, in one country, with an add-on design (all took an antidepressant); sample size and funding are not in the abstract; the main outcome (remission) did not differ. Funding and conflicts: Funding is not stated in the abstract. Registration: ChiCTR2300071421 Read: Abstract read through PubMed (esummary and efetch) on 2026-10-31; full text not read.
Liu 2019 Prebiotics and probiotics for depression and anxiety: A systematic review and meta-analysis of controlled clinical trials. Neurosci Biobehav Rev · PMID 31004628 · DOI	Systematic review and meta analysis of controlled trials	34 controlled clinical trials of prebiotics or probiotics for depression or anxiety, in community, medical and psychiatric samples	Probiotics or prebiotics (many products and strains) Varied by trial · vs Placebo	Probiotics gave small effects on depression and anxiety; prebiotics did not differ from placebo. The depression effect was larger in clinical or medical samples and, in a preliminary analysis of psychiatric samples only, medium to large. The authors say there were few trials in clinical samples. Probiotics: depression d = -0.24, anxiety d = -0.10. Prebiotics: depression d = -0.08, anxiety d = 0.12. Clinical or medical samples: d = -0.45. Psychiatric samples (preliminary): d = -0.73. (Not given; p < .01 (probiotics, depression), p = .03 (probiotics, anxiety), p = .51 and p = .11 (prebiotics)) Methods, limits and funding Outcomes: Depression and anxiety scores Limitations: Few trials in people with a diagnosis; the psychiatric-sample result was preliminary. Funding and conflicts: Funding is not stated in the abstract. Read: Abstract read through PubMed (esummary and efetch) on 2026-10-31; full text not read.
Masamura 2024 Differences in the human gut microbiota with varying depressive symptom severity scores. Biosci Microbiota Food Health · PMID 39364125 · DOI	Cross sectional analysis of commercial test data	People in Japan who used commercial gut microbiota testing services and completed a depression-tendency questionnaire (number not given in the abstract)	Commercial stool microbiome test data One time point · vs Groups by depression screening score	No significant differences in gut microbiome composition were found among the groups. A few bacterial families (Bifidobacteriaceae, Streptococcaceae, Veillonellaceae) were candidates for differences. The authors note that participants' diagnoses were not clear. No significant composition differences (Not given in the abstract) Methods, limits and funding Outcomes: Gut microbiome composition by depression screening score Limitations: No clinical diagnoses; screening questionnaire only; sample size not in the abstract. Funding and conflicts: Funding is not stated in the abstract. Read: Abstract read through PubMed (esummary and efetch) on 2026-10-31; full text not read.
Minkoff 2023 Fecal microbiota transplantation for the treatment of recurrent Clostridioides difficile (Clostridium difficile). Cochrane Database Syst Rev · PMID 37096495 · DOI	Cochrane systematic review of randomized trials	320 6 randomized trials of FMT for recurrent C. difficile infection in immunocompetent adults, 320 participants	Donor-based FMT (nasoduodenal tube, enema or colonoscopy) Varied · vs Placebo, autologous FMT, no intervention or antibiotics	FMT likely led to a large increase in resolution of the infection (moderate certainty). The review found no conclusive evidence on safety because serious events were few, and could draw no conclusions for immunocompromised people. Resolution RR 1.92; serious adverse events RR 0.73 (Resolution 1.36 to 2.71; serious adverse events 0.38 to 1.41) Methods, limits and funding Outcomes: Resolution of infection; serious adverse events Limitations: Only 6 trials; people with severely weakened immunity were mostly excluded. Funding and conflicts: Funding is not stated in the abstract. Read: Abstract read through PubMed (esummary and efetch) on 2026-10-31; full text not read.
Ng 2023 Effect of Probiotic Supplementation on Gut Microbiota in Patients with Major Depressive Disorders: A Systematic Review. Nutrients · PMID 36986088 · DOI	Systematic review (no meta analysis)	7 clinical trials in people with major depressive disorder	Probiotic supplements Mostly 4 to 8 weeks · vs Placebo (one trial was open-label)	Probiotics gave only modest effects on depressive symptoms. There were no consistent effects on gut microbiota diversity and, in most cases, no significant change in composition after four to eight weeks. Adverse events were not reported systematically and there were no good longer-term data. Not pooled (Not pooled) Methods, limits and funding Outcomes: Depressive symptoms; gut microbiota diversity and composition Limitations: Small and varied set of trials; most did not control for the effect of diet on gut microbes. Funding and conflicts: Funding is not stated in the abstract. Read: Abstract read through PubMed (esummary and efetch) on 2026-10-31; full text not read.
Nikolova 2021 Perturbations in Gut Microbiota Composition in Psychiatric Disorders: A Review and Meta-analysis. JAMA Psychiatry · PMID 34524405 · DOI	Systematic review and meta analysis of case control studies	59 case-control studies of adults with depression, bipolar disorder, psychosis and schizophrenia, anorexia nervosa, anxiety, obsessive compulsive disorder, PTSD or ADHD; 34 studies (1,519 patients, 1,429 controls) in the diversity meta-analyses	Stool microbiome measures (diversity and relative abundance of gut microbes) Cross-sectional comparison · vs People without a psychiatric diagnosis	Richness was lower in patients overall (observed species SMD -0.26; Chao1 SMD -0.5) but consistently lower only in bipolar disorder. Depleted Faecalibacterium and Coprococcus and enriched Eggerthella were shared across depression, bipolar disorder, schizophrenia and anxiety, with little specificity to one disorder. Observed species SMD -0.26; Chao1 SMD -0.5; phylogenetic diversity SMD -0.24 (Observed species -0.47 to -0.06; Chao1 -0.79 to -0.21; phylogenetic diversity -0.47 to -0.001) Methods, limits and funding Outcomes: Alpha and beta diversity; relative abundance of gut microbes Limitations: Case-control design cannot show cause. The abstract says the confounding roles of region and medication were evaluated but does not give the result. Funding and conflicts: Funding is not stated in the abstract. Read: Abstract read through PubMed (esummary and efetch) on 2026-10-31; full text not read.
Porcari 2025 International consensus statement on microbiome testing in clinical practice. Lancet Gastroenterol Hepatol · PMID 39647502 · DOI	International expert consensus statement	International multidisciplinary expert panel	Microbiome testing in clinical practice No comparison group · vs No comparison group	The statement says evidence supporting the clinical usefulness of microbiome diagnostics is scarce, and that commercial providers offer direct-to-consumer diagnostic tests without a consensus on regulation or any proven value in clinical practice. It sets out a framework for best practice. No effect size reported (No interval reported) Methods, limits and funding Outcomes: Recommendations for testing; evidence for clinical usefulness Limitations: Expert consensus, not a trial; the abstract does not give the number of experts. Funding and conflicts: Funding is not stated in the abstract. Read: Abstract read through PubMed (esummary and efetch) on 2026-10-31; full text not read.
Rapoport 2022 Adverse events in fecal microbiota transplantation: a systematic review and meta-analysis. Ann Gastroenterol · PMID 35479587 · DOI	Systematic review and meta analysis	5099 61 studies of FMT for recurrent Clostridioides difficile infection, 5,099 patients	Fecal microbiota transplantation Varied · vs Not pooled against a control	Serious adverse events related to FMT occurred in less than 1% of patients; serious events not related to FMT were 2.9%. Minor adverse events were infrequent, self-limited gut and body discomfort. SAEs related to FMT below 1%; unrelated 2.9% (Not given in the abstract) Methods, limits and funding Outcomes: Serious adverse events Limitations: Includes mostly uncontrolled studies in a specific infection; says little about use for depression. Funding and conflicts: Funding is not stated in the abstract. Read: Abstract read through PubMed (esummary and efetch) on 2026-10-31; full text not read.
Ribera 2024 Probiotic, prebiotic, synbiotic and fermented food supplementation in psychiatric disorders: A systematic review of clinical trials. Neurosci Biobehav Rev · PMID 38280441 · DOI	Systematic review of clinical trials	47 articles from 42 studies in people with a psychiatric diagnosis (major depression 19 studies, schizophrenia 11, bipolar disorder 5, generalized anxiety disorder 1 and others)	Probiotics, prebiotics, synbiotics or fermented foods Varied · vs Placebo or usual care (varied)	Except in major depression, current evidence does not clarify the role of probiotics and prebiotics in treating mental illness. Several studies pointed to improvement in markers such as CRP and IL-6. Not pooled (Not pooled) Methods, limits and funding Outcomes: Psychiatric symptoms and inflammatory markers Limitations: Only one study in generalized anxiety disorder; lifestyle and diet were possible confounders. Funding and conflicts: Funding is not stated in the abstract. Read: Abstract read through PubMed (esummary and efetch) on 2026-10-31; full text not read.
Servetas 2026 Evaluating the analytical performance of direct-to-consumer gut microbiome testing services. Commun Biol · PMID 41748906 · DOI	Laboratory evaluation (method comparison)	7 Seven direct-to-consumer gut microbiome testing services tested with a standardized NIST-developed human fecal material	Direct-to-consumer stool test services Single evaluation · vs The standard material and between-provider comparison	The tests showed major discrepancies within and across providers. Variability between providers was on the same scale as biological variability between donors. The authors attribute this to methodological variability and lack of quality control. Between-provider variability similar in size to between-donor variability (Not given) Methods, limits and funding Outcomes: Analytical performance (do tests agree?) Limitations: Tested standard material, not people; only seven services. Funding and conflicts: Funding is not stated in the abstract. Read: Abstract read through PubMed (esummary and efetch) on 2026-10-31; full text not read.
Simpson 2021 The gut microbiota in anxiety and depression - A systematic review. Clin Psychol Rev · PMID 33271426 · DOI	Systematic review	26 studies: 2 case-control comparisons in generalised anxiety disorder, 18 in depression, 1 in both, 5 symptom-only	Gut microbiota comparisons Cross-sectional · vs People without the condition	Alpha and beta diversity findings were inconsistent. Differences in taxa pointed to more pro-inflammatory species (for example Enterobacteriaceae and Desulfovibrio) and fewer short-chain fatty acid producers (for example Faecalibacterium). The authors say future research should assess confounders, particularly diet and psychotropic medicines. Not given (Not given) Methods, limits and funding Outcomes: Diversity and bacterial taxa in anxiety and depression Limitations: Only 2 case-control studies in generalised anxiety disorder; diet and medicines not well controlled. Funding and conflicts: Funding is not stated in the abstract. Read: Abstract read through PubMed (esummary and efetch) on 2026-10-31; full text not read.
Valles-Colomer 2019 The neuroactive potential of the human gut microbiota in quality of life and depression. Nat Microbiol · PMID 30718848 · DOI	Population cohort with validation cohorts	1054 Flemish Gut Flora Project (1,054 people) with validation in independent data sets (1,070 in total)	Stool metagenomes and questionnaires on quality of life and depression Cross-sectional · vs Comparison across people	Butyrate-producing Faecalibacterium and Coprococcus were associated with higher quality of life. Coprococcus and Dialister were also depleted in depression, even after correcting for antidepressants. Not given as one number in the abstract (Not given in the abstract) Methods, limits and funding Outcomes: Gut microbial features linked with quality of life and depression Limitations: A link, not proof of cause. The authors themselves stress the importance of confounders. Funding and conflicts: Funding is not stated in the abstract. Read: Abstract read through PubMed (esummary and efetch) on 2026-10-31; full text not read.
Wang 2026 Case Series of Bacteremia Associated with Probiotic Use in Children after Cardiac Surgery, China. Emerg Infect Dis · PMID 41612523 · DOI	Case series within a hospital cohort	16436 16,436 children who had cardiac surgery in China (2019 to 2024); 5,034 received probiotics	Perioperative probiotics No comparison group · vs No comparison group	Six children developed bloodstream infection with probiotic strains (Bacillus subtilis, Bacillus licheniformis, Lacticaseibacillus rhamnosus). Three of them had not received probiotics directly, suggesting cross-contamination or catheter-related spread. All six had complex congenital heart disease and central venous catheters. Antibiotics cleared the blood cultures in all; one child died of the underlying heart disease. 6 cases among 5,034 children given probiotics (probiotic-strain bacteremia) (Not given) Methods, limits and funding Outcomes: Bacteremia with probiotic strains Limitations: Observational, a very specific group of severely ill children; the authors say the infection is rare. Funding and conflicts: Funding is not stated in the abstract. Read: Abstract read through PubMed (esummary and efetch) on 2026-10-31; full text not read.
Zhang 2025 Clinical efficacy of fecal microbiota transplantation in alleviating depressive symptoms: a meta-analysis of randomized trials. Front Psychiatry · PMID 41122746 · DOI	Systematic review and meta analysis of randomized trials	681 12 randomized trials with 681 participants, including people with irritable bowel syndrome and people with neurological or psychiatric conditions	Fecal microbiota transplantation (capsule, endoscopic or enema routes) Effects short to mid term; diminished by 6 months · vs Control conditions	FMT reduced depressive symptoms overall. Direct delivery into the gut looked stronger than capsules. Benefits were most notable in the short to mid term and diminished by 6 months. The effect was stronger in people with irritable bowel syndrome (SMD -1.06) than in people with neurological or psychiatric conditions (SMD -0.67). SMD -1.21; sensitivity analysis SMD -0.56; IBS -1.06; neurological or psychiatric -0.67; I2 47% (-1.87 to -0.55; sensitivity -0.86 to -0.26) Methods, limits and funding Outcomes: Depressive symptoms Limitations: Mixed populations (many with gut disease, not depression); different procedures; effects faded by 6 months; small trials. Funding and conflicts: Funding is not stated in the abstract. Registration: PROSPERO CRD42025638185 Read: Abstract read through PubMed (esummary and efetch) on 2026-10-31; full text not read.
p23759244 Psychobiotics: a novel class of psychotropic. (Biol Psychiatry 2013) · Source	Narrative review human and animal			Methods, limits and funding Read: abstract
p27793434 Psychobiotics and the Manipulation of Bacteria-Gut-Brain Signals. (Trends Neurosci 2016) · Source	Narrative review human and animal			Methods, limits and funding Read: abstract
p31460832 The Microbiota-Gut-Brain Axis. (Physiol Rev 2019) · Source	Narrative review human and animal			Methods, limits and funding Read: abstract
p31978342 Establishing or Exaggerating Causality for the Gut Microbiome: Lessons from Human Microbiota-Associated Rodents. (Cell 2020) · Source	Systematic review of animal studies animal studies reviewed			Methods, limits and funding Read: abstract
s-fda-vowst FDA: VOWST (content current as of 05/18/2023) · Source	Regulator n/a			Methods, limits and funding Read: full-text
s-nccih-probiotics NCCIH: Probiotics: Usefulness and Safety (Last Updated August 2019) · Source	Health agency n/a			Methods, limits and funding Read: full-text
s-nimh-depression NIMH: Depression · Source	Health agency n/a			Methods, limits and funding Read: full-text
s-nimh-help NIMH: Help for Mental Illnesses · Source	Health agency n/a			Methods, limits and funding Read: full-text

What we do not know yet

No trial read tested one named probiotic strain against placebo in a large, independent, long-term study in people with a diagnosed anxiety disorder.
No review read compared probiotic trials by strain with enough trials per strain to say which strains work.
Trials are short (mostly up to 8 weeks); no long-term follow-up was read.
Only one randomized FMT trial in major depression was read, and its main outcome (remission) did not differ.
No validated stool test for depression or anxiety was found.
Safety data for probiotics in pregnancy and breastfeeding were not read.

How new research is added

New studies are found by a saved PubMed search and placed in a review queue before the page changes.

Lab markers

What changed in blood or saliva tests during trials. A changed number is not the same as feeling or being healthier, and we give no "normal ranges".

Other 6

●

Gut microbe composition (stool microbiome)

Group differences, not consistent: often fewer butyrate-producing bacteria (Faecalibacterium, Coprococcus) and more of some pro-inflammatory groups (such as Eggerthella); diversity results conflicted. in trials

The mix and variety of microbes found in a stool sample, usually measured by gene sequencing.

Mixed findings

Results and what it means

Why it was measured

To see whether people with depression or anxiety have different gut microbes from people without.

Statistical vs. clinical meaning

These are group averages seen at one moment. They do not show that microbes cause low mood, and they cannot be used to diagnose an individual.

Study	Result	People	Product
¹⁸	Depleted Faecalibacterium and Coprococcus and enriched Eggerthella shared across diagnoses; richness lower overall but consistent only in bipolar disorder	59 case-control studies; 1,519 patients and 1,429 controls in diversity analyses	Stool microbiome comparison
²⁴	Coprococcus and Dialister depleted in depression, also after correcting for antidepressants	1,054 people with validation in independent data sets	Stool metagenomes
²³	Diversity inconsistent; more pro-inflammatory and fewer short-chain fatty acid producing bacteria	26 studies, mostly depression	Stool microbiome comparison
⁴	Diversity inconsistent; conflicting taxa results because of diet and medicine confounders	24 articles, 20 from China	Stool microbiome comparison

^{18 24 23 4}

●

Depression symptom scores

Lower scores with probiotics and diet support in several trials, with large differences between reviews. in trials

Rating scales (for example MADRS) that score how severe depressive symptoms are.

Low certainty

Results and what it means

Why it was measured

The main outcome in trials of probiotics and diet.

Statistical vs. clinical meaning

Scale scores matter to patients but self-reported scales can be affected by knowing which treatment you receive, and the trials were short.

Study	Result	People	Product
¹⁴	Probiotics d -0.24 vs placebo; prebiotics no difference	34 controlled trials	Probiotics or prebiotics
¹	Probiotics SMD -0.96	23 trials, 1,401 patients	Probiotics
⁷	Probiotic monotherapy SMD -0.38; not significant after excluding industry-funded trials	6 trials, 341 adults	Probiotics
¹⁰	MADRS fell more with dietary support (d -1.16)	67 adults	Dietitian-led diet support

^{14 1 7 10}

●

Anxiety symptom scores

Small to moderate reductions with probiotics in two meta-analyses. in trials

Rating scales that score how severe anxiety symptoms are.

Very low certainty

Results and what it means

Why it was measured

A secondary outcome in many trials of probiotics.

Statistical vs. clinical meaning

Anxiety was a side outcome in trials built around depression.

Study	Result	People	Product
¹⁴	Probiotics d -0.10; prebiotics d 0.12 (not significant)	34 controlled trials	Probiotics or prebiotics
¹	Probiotics SMD -0.59	9 trials of probiotics for anxiety	Probiotics

^{14 1}

●

Perceived stress score

Fell in both diet groups; the psychobiotic diet did not differ significantly from control. in trials

A questionnaire score of how stressed a person feels.

Not enough evidence

Results and what it means

Why it was measured

Used to test diets and probiotics in healthy people.

Statistical vs. clinical meaning

Self-reported, short trial in healthy people.

Study	Result	People	Product
²	32% fall on psychobiotic diet vs 17% on control; difference not significant	45 healthy adults	Psychobiotic diet

●

Remission and HAMD-17 depression score

No significant remission difference at week 8; greater HAMD-17 reduction at weeks 2 and 8 with FMT. in trials

Remission is a score below a set cut-off; HAMD-17 is a clinician-rated depression scale.

Not enough evidence

Results and what it means

Why it was measured

Clinical outcomes in the FMT trial.

Statistical vs. clinical meaning

One trial, add-on design, null main outcome.

Study	Result	People	Product
¹³	Remission not significantly different; HAMD-17 fell more with FMT	Patients with major depressive disorder taking an antidepressant	FMT capsules vs placebo

¹³

●

Accuracy of microbiome-based classifiers (AUC)

Moderate accuracy in one research study; not tested in independent groups in the sources read. in trials

AUC measures how well a model separates two groups; 0.5 is a coin flip and 1.0 is perfect.

Very low certainty

Results and what it means

Why it was measured

To judge whether a stool test could diagnose or predict treatment response.

Statistical vs. clinical meaning

A modest AUC in one study does not make a usable clinical test.

Study	Result	People	Product
¹²	AUC 0.733 patients vs controls; 0.800 responders vs non-responders	62 patients with bipolar depression, 60 controls	Random forest on stool metagenomes

¹²

Safety

What studies and official sources say about side effects, medicines and who should check first. Each point shows what kind of evidence it comes from.

Main points to know 5

This is not a replacement for mental health care. Persistent low mood, anxiety, thoughts of self-harm, or big changes in sleep or appetite need assessment by a health professional. NIMH describes depression as an illness with severe symptoms that affect how you feel, think and handle daily activities, and says treatment usually includes therapy, medication, or both. NCCIH says not to use probiotics as a reason to postpone seeing your health care provider about any health problem and to consult one first if you have health problems. Do not stop, reduce or replace prescribed medicines for a probiotic, diet or test result without talking to the prescriber. (The advice not to change medicines without the prescriber is general safety guidance, not a quoted source.) Health agency guidance ^{33 34 32}
If you are thinking about harming yourself, get help now. If you are thinking about ending your life, or you cannot keep yourself safe, contact your local emergency services or a crisis line in your country right away, or go to the nearest emergency department, and tell someone you trust. Microbiome products and tests are not a treatment for a mental health crisis. NIMH says that in the United States, in life-threatening situations you call 911, and that anyone struggling or having thoughts of suicide can call or text 988 (Suicide & Crisis Lifeline). Elsewhere, use your local emergency number or national crisis service. Health agency guidance ^{34 33}
Probiotics can cause serious infections in people who are very ill or have weakened immunity. NCCIH says probiotics have an extensive history of apparently safe use, particularly in healthy people, but the risk of harmful effects is greater in people with severe illnesses or compromised immune systems. Cases of severe or fatal infections have been reported in premature infants, and the FDA has warned health care providers about this. In a Chinese cohort of 16,436 children after heart surgery, six developed bloodstream infection with probiotic strains, and all six had central venous catheters; in an 11-year review at one children's hospital there were 8 Lactobacillus bloodstream infections among 127,845 admissions, most in patients with impaired gut function and a central line. People with weakened immune systems, a central line, a serious illness or a premature baby should only use probiotics if their clinician decides it is right. Health agency guidance and case series ^{32 25 8}
Fecal microbiota transplant is a medical procedure with real risks. FMT is used by clinicians for recurrent Clostridioides difficile infection. In a Cochrane review of 6 trials (320 adults) it likely led to a large increase in resolution of that infection, but the review found no conclusive evidence on safety. In two clinical trials, two patients developed drug-resistant E. coli bloodstream infection linked to the same stool donor, and one of them died. A pooled analysis in C. difficile patients found serious events related to FMT in less than 1%. The FDA page for VOWST lists its indication as preventing recurrence of C. difficile infection in adults after antibacterial treatment, not depression. Do-it-yourself transplants are not safe and have no place here. Systematic reviews, case report and regulator page ^{16 6 20 31}
Consumer microbiome tests cannot diagnose depression or anxiety. An international expert panel says commercial microbiome diagnostic tests are offered without consensus on regulation or any proven value in clinical practice. A test of seven services using the same standard stool material found major discrepancies within and across providers, as large as the biological differences between donors. A result may also lead to unneeded supplements, diets or worry, or to false reassurance and delay in seeking care. Expert consensus and laboratory comparison ^{19 22 15}

Side effects 7

Effect	How often	Severity and recovery	Evidence
Bloodstream infection with probiotic bacteria or yeast Risk groups named: central venous catheter, impaired gut function, premature infants, weakened immunity (NCCIH).	6 cases among 5,034 children given probiotics after heart surgery; 8 Lactobacillus cases in 127,845 pediatric admissions over 11 years	Serious (fever, systemic infection); treated with antibiotics in the series read Cleared with antibiotics in all six children in the heart-surgery series; one child died from the underlying heart disease	Case series ^{25 8}
Severe or fatal infection in premature infants given probiotics NCCIH says the FDA warned health care providers about this risk.	Frequency not given on the page	Severe, potentially fatal Not stated	Health agency guidance ³²
Other possible harms of probiotics: harmful substances produced by the organisms and transfer of antibiotic resistance genes NCCIH says few studies have looked at the safety of probiotics in detail, so the frequency and severity of side effects is not well known.	Not stated	Not stated Not stated	Health agency guidance ³²
Minor side effects in probiotic trials A 2023 review found adverse events were not reported systematically in the depression trials.	No difference from placebo in a 6-trial analysis	Minor Not stated	Systematic review of randomized trials ^{7 17}
Bloodstream infection from a drug-resistant bacterium carried in donor stool (FMT) The authors call for stronger donor screening.	Two cases reported linked to one donor	Serious; one death Not stated	Case report ⁶
Minor gut and body discomfort after FMT Serious events related to FMT in this infection were below 1%; this does not apply automatically to people using FMT for depression.	Infrequent in a pooled analysis of 5,099 patients with C. difficile infection	Minor, self-limited Self-limited	Systematic review and meta analysis ²⁰
Product contents not matching the label NCCIH says some probiotic products have been reported to contain microorganisms other than those listed on the label.	Not stated	Some contaminants may pose serious health risks Not stated	Health agency guidance ³²

Medicines and supplements 3

With	Why it matters	Importance	Evidence	What to do
Psychiatric medicines and antidepressants prescription	No interaction study with probiotics, prebiotics or diets was read. The 2026 FMT trial gave FMT capsules alongside a standard antidepressant and reported tolerability comparable to placebo.	No interaction between probiotics and psychiatric medicines was found in the sources read. Absence of a study is not proof of safety.	Absence of evidence Not enough evidence ¹³	Tell your prescriber about any probiotic, supplement or diet change, and do not change a prescribed medicine on your own.
Medicines that weaken the immune system, chemotherapy and other immune suppression prescription	NCCIH says the risk of harmful effects from probiotics is greater in people with severe illnesses or compromised immune systems.	Live bacteria or yeast in a probiotic may cause infection when immunity is weak.	Health agency guidance Moderate certainty ³²	Ask the clinician who treats you before taking any probiotic if your immune system is weakened.
Antibiotics prescription	NCCIH lists transfer of antibiotic resistance genes from probiotic organisms to other gut microbes as a possible harm; the part read gives no rule on timing with antibiotics.	No source read gives guidance on combining probiotics with antibiotics for mood.	Absence of evidence Not enough evidence ³²	Ask a doctor or pharmacist.

Who should check first 5

Situation	Advice	Evidence
Weakened immune system, serious illness, a central venous catheter or recent major surgery	Ask the treating clinician before using any probiotic. NCCIH says the risk of harm is greater in these groups.	Health agency guidance and case series ^{32 25 8}
Premature infants	Not for self-treatment. NCCIH says severe or fatal infections have been reported and the FDA has warned health care providers.	Health agency guidance ³²
Thoughts of self-harm, severe depression or a mental health crisis	Seek urgent professional help. A microbiome product or test is not a treatment for a crisis.	Health agency guidance ³⁴
Fecal transplant outside a clinical setting	Do not try it yourself. In two trials donor stool carried a drug-resistant bacterium and one patient died.	Case report ⁶
Planning to stop or replace a prescribed treatment because of a probiotic, diet or test result	Talk to the prescriber first (general safety guidance).	General safety guidance

Overdose, product quality and regulators

Body	Action	Date
National Center for Complementary and Integrative Health (NCCIH, US) (United States)	Probiotics page: in most instances we still do not know which probiotics are helpful and which are not; risk of harm is greater in people with severe illness or weakened immunity; severe or fatal infections reported in premature infants and the FDA warned providers; do not use probiotics as a reason to postpone seeing a provider; probiotic supplements do not need FDA approval before they are marketed. source	2019-08
US Food and Drug Administration (United States)	VOWST (fecal microbiota spores, live-brpk) is indicated to prevent recurrence of Clostridioides difficile infection in adults after antibacterial treatment for recurrent infection. It is not listed for depression or any mental health condition. source	2023-05-18
National Institute of Mental Health (NIMH, US) (United States)	Help page: in life-threatening situations call 911; if suicidal or in emotional distress consider the 988 Suicide & Crisis Lifeline by call, text or chat; a primary care provider can do an initial mental health screening and refer. source	Not dated on the page

When to get medical help

Thoughts of harming yourself or ending your life, or feeling you cannot keep yourself safe: contact local emergency services or a crisis line in your country right away, or go to the nearest emergency department, and tell someone you trust. In the US, NIMH lists 911 for emergencies and 988 (call or text) for crisis support.
Fever, chills, a fast heartbeat, confusion or feeling very unwell after starting a probiotic or after a fecal transplant, especially with a weakened immune system, a central line or a serious illness: seek urgent medical help. Fever was common in the case series read; the rest of this wording is general guidance.
Low mood, anxiety, loss of interest or changes in sleep or appetite that last more than a couple of weeks or get in the way of daily life: see a doctor or mental health professional (general guidance, consistent with NIMH).
Severe or bloody diarrhea, severe abdominal pain or persistent vomiting after a probiotic or FMT: contact a clinician promptly (general safety guidance, not a quoted source).

Stop using the product and contact a clinician or emergency service. In the US, Poison Control is 1-800-222-1222.

General information only, not medical advice. Ask a clinician or pharmacist before use, especially with medicines, in pregnancy or with a health condition. See the Natural Remedy Safety guide.

Frequently asked questions

Does gut bacteria cause depression?

Not shown. Studies find group differences, but most are case-control and affected by diet and medicines, and group differences cannot show the direction of cause.

^{18 23 4}

What is the gut-brain axis?

A 2019 review describes two-way communication between gut microbes and the brain through the immune system, tryptophan metabolism, the vagus nerve and the gut nervous system. Much of the detail comes from animals.

²⁹

Do probiotics help depression?

Some reviews report small to large symptom drops, but results vary a lot between reviews, trials are short and one analysis lost its effect without industry-funded trials. They are not an established treatment.

^{14 1 7}

Do probiotics help anxiety?

Effects were very small to moderate in two meta-analyses, and almost no trials were in diagnosed anxiety disorders.

^{14 1 21}

What are psychobiotics?

A term from 2013 for live organisms that, taken in adequate amounts, give a health benefit in people with psychiatric illness; widened in 2016 to include prebiotics.

^{27 28}

Do prebiotics or fibre help mood?

Prebiotics did not differ from placebo for depression or anxiety in the 2019 meta-analysis, and the 2025 analysis found no significant effect on depression.

^{14 1}

Does a Mediterranean-style diet help depression?

The SMILES trial and a five-trial meta-analysis found lower depression scores, but both are small or very varied and the meta-analysis rated certainty low. It is a supportive step, not a replacement for care.

^{10 3}

Do fermented foods help anxiety or stress?

Not shown. One four-week diet trial in healthy adults found no significant difference from control, and a survey cannot show cause.

^{2 9}

Is a fecal transplant a treatment for depression?

No. It is being tested. A 2026 trial added to an antidepressant did not show a different remission rate at week 8, and the FDA lists VOWST for preventing recurrent C. difficile infection.

^{13 31}

Can a gut microbiome test tell me if I am depressed or anxious?

No. No validated test was found, a commercial dataset showed no composition differences by screening score, and an expert panel says commercial tests lack proven clinical value.

^{15 19 22}

Who should avoid probiotics or ask a doctor first?

People with weakened immune systems, serious illness, central venous catheters, and premature infants. NCCIH says the risk of harm is greater in these groups.

^{32 25 8}

Should I take a probiotic instead of my medicine or therapy?

No. Talk with the prescriber before changing any medicine. NCCIH says not to use probiotics as a reason to postpone seeing a provider. If you are in crisis, contact local emergency services or a crisis line.

^{32 34}

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Editorial transparency

Written by: AlternativeMed editorial team. Research compiled with AI assistance, then checked against PubMed records.
Medical review: None. This page has not been reviewed by a physician or pharmacist.
First published: 2026-10-31
Last substantive update: 2026-10-31
How we research: We searched PubMed, read the abstract of every review, trial and study cited, and read the NCCIH, FDA and NIMH pages as downloaded text. Full texts were not read. Certainty ratings are the compiler's own reading unless a review gives its own grade (stated in the text). Funding is not stated in most abstracts, and this is noted where relevant.
How we rate evidence: See "How we rate evidence" in the Evidence tab.
Independence: No product is promoted on this page. No affiliate links are shown.

Sources 34

Each source was opened and checked. Where we read only the abstract, it says so. The small numbers in the text point here.

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Education, not medical advice. This page is for general information. It does not diagnose or treat any condition and is not a substitute for care from a qualified health professional. Natural does not automatically mean safe. Ask a clinician or pharmacist before changing your care, especially if you take medication, are pregnant or breastfeeding, or have a health condition.