Microbiome Research and Mental Health

If you or someone you know is in crisis

In the US, call or text 988 (Suicide & Crisis Lifeline), or call 911 for an emergency. Elsewhere, contact local emergency services. Microbiome products are not a treatment for a mental health crisis.

Short answer

Studies do find differences in gut microbes between groups of people with and without depression or anxiety. They do not show that microbes cause these conditions in people, and a microbiome test cannot diagnose them. Probiotics have shown small to moderate effects on symptoms in some reviews, with very uneven results between reviews and short follow-up. Fecal transplant for depression is at the trial stage.

Evidence labels on this page: association only, preclinical (cells/animals only), mixed, promising but limited and insufficient evidence. See How We Label Evidence in Specialty Topics.

What observational studies show

Association only. A 2021 review and meta-analysis of 59 case-control studies covered depression, bipolar disorder, psychosis and schizophrenia, anxiety and other conditions. It found a pattern shared across diagnoses: lower levels of some butyrate-producing bacteria (Faecalibacterium and Coprococcus) and higher levels of Eggerthella. Little evidence was found that these were specific to one condition. [1] A large population study of 1,054 people found Coprococcus and Dialister depleted in depression even after correcting for antidepressants. [2]

The main weakness is confounding. Reviews of anxiety and depression repeatedly call for better handling of diet and psychotropic medication, which also change gut microbes. [3] Another 2025 review found conflicting results on bacterial abundance because of such confounders, and 20 of its 24 studies were conducted in China. [4] Diversity findings were inconsistent. [3][4]

A group difference also cannot tell you the direction of cause: microbes might contribute to mood, mood and its effects on sleep, diet and medicines might change microbes, or both. See Microbiome Studies: Association vs Causation.

Animal and "psychobiotic" research

The term "psychobiotic" was introduced in 2013 for live organisms that, taken in adequate amounts, produce a health benefit in people with psychiatric illness. At that time evidence of antidepressant or anxiolytic activity came from rodents, with human results awaited. [5] In 2016 the definition was widened to include prebiotics, and the authors noted that knowledge of how the microbiome responds to outside influence remains limited. [6] Label: preclinical (cells/animals only) for the animal findings. Transfer of microbes from people into rodents reports disease features in 95% of studies, which the authors called implausibly high. [7] See The Gut-Brain Axis: What Research Shows.

Probiotics and symptoms in human trials

Label: mixed, leaning promising but limited.

The three reviews differ in size of effect, partly because they included different trials and populations. The authors of the 2023 review noted that trials largely did not control for diet's effect on gut microbes. [9] Strains, doses and durations also vary. NCCIH says that in most instances we still do not know which probiotics are helpful and which are not. [11] For ingredient-level guides, see Probiotic Supplements Explained.

A diet trial in 45 healthy adults found perceived stress fell 32% on a "psychobiotic" diet (high in prebiotic and fermented foods) versus 17% on the control diet over 4 weeks, but the difference between the groups was not significant. [12] Label: insufficient evidence for stress benefit from that diet.

Fecal microbiota transplant for depression

In a 2026 randomized, double-blind, placebo-controlled trial, patients with major depressive disorder received fecal microbiota capsules or placebo for two weeks alongside escitalopram. Remission rates at week 8 did not differ significantly, although depression scale scores fell more in the transplant group at weeks 2 and 8. The authors reported tolerability comparable to placebo. [13] This is one trial; label: insufficient evidence pending replication. The FDA-approved fecal-based products named on its site, REBYOTA and VOWST, are indicated to prevent recurrence of Clostridioides difficile infection after antibiotic treatment, not for depression. [14][15] See Fecal Microbiota-Based Therapies.

Can a stool test detect depression?

Proper evidence not found for a validated stool test that diagnoses or screens for depression. A commercial-test dataset from Japan found no significant differences in gut microbiome composition among groups based on depression screening scores. [16] Machine-learning "signatures" have been published in research, for example a random forest model in 62 patients with bipolar depression and 60 healthy people (area under the curve 0.733), but the searches for this page found no independent validation in a clinical setting. [17] See Commercial Stool Tests.