Probiotic Supplements Explained: Probiotics, Prebiotics, Synbiotics, Postbiotics and Fermented Foods
A plain-language starting guide to probiotic supplements: the key terms, what research found, why strain matters, who should ask a clinician first, and where to read next.
Content last updated . This page has not been medically reviewed.
Quick answer
A probiotic supplement is a product sold with live microorganisms, usually bacteria or a yeast. Whether it helps depends on the exact strain, the health question and the person. The American Gastroenterological Association (AGA) reviewed eight digestive situations in a 2020 technical review. In most of them it found either too little evidence to recommend probiotics or a major knowledge gap. [1] The NHS (UK) says there is little evidence to support many of the health claims made about probiotics. [2]
This page is the starting guide to the whole Probiotics & Gut Support library. It is information only, not medical advice.
The words, in one place
The formal definitions come from ISAPP. Our page on probiotics, prebiotics and fermented foods explains them in everyday language, and prebiotics, synbiotics and postbiotics defined goes deeper.| Term | Plain meaning | Source |
|---|---|---|
| Probiotic | Live microorganisms that, given in adequate amounts, give the host a health benefit | [6] |
| Prebiotic | A substrate that host microorganisms use selectively, giving a health benefit; the benefit must be documented | [7] |
| Synbiotic | A mixture of live microorganisms and substrate(s) that host microorganisms use selectively, giving a health benefit | [8] |
| Postbiotic | A preparation of inanimate (not living) microorganisms and/or their components that gives a health benefit | [9] |
| Fermented food | Food made through desired microbial growth and enzyme changes to its ingredients | [10] |
So a fermented food is not automatically a probiotic, because the two terms cannot be used interchangeably. [10] Not all dietary fibers are prebiotics. [11] Heat-killed bacteria do not meet the strict definition of a probiotic; the AGA noted that several "L. acidophilus" diarrhea trials had tested heat-killed preparations. [1]
Supplement, food or medicine?
- Food. Yogurt, kefir and similar foods are a separate category. See fermented foods vs probiotic supplements. Results for a capsule do not transfer to a food, or the reverse.
- Supplement. In the US, depending on its intended use, the FDA might regulate a probiotic as a dietary supplement, a food ingredient or a drug. Many are sold as supplements, which do not need FDA approval before they are marketed. [5] In the UK the NHS says probiotics are generally classed as food rather than medicine. It adds that we cannot always be sure a product contains the bacteria on the label, enough of them to have an effect, or bacteria that survive long enough to reach the gut. [2] NCCIH notes that some products have been reported to contain organisms other than those on the label. [5] See quality testing and certifications.
- Medicine. The AGA points out that none of the probiotic preparations being studied are currently manufactured as drugs. [1] The NHS says there is likely a huge difference between the pharmaceutical-grade probiotics that show promise in trials and the yogurts and supplements sold in shops. [2]
What the evidence looks like
Why strain matters
NCCIH says that if one kind of Lactobacillus helps prevent an illness, that does not mean another kind of Lactobacillus or any Bifidobacterium would do the same. [5] A systematic review of 228 trials found evidence for both strain specificity and disease specificity. It warned that pooling different probiotics together can give misleading conclusions. [14] Learn to read a label on probiotic strains and labels, and see why one strain's result does not transfer.Amounts and official intake levels
The sources we read do not describe an official recommended intake (RDA, AI or upper limit) for probiotics. Amounts used in trials are study amounts for one product and one group of people, not advice. A clinician or pharmacist can help with what suits a particular person.A cautious way to decide
- Name the goal. "Gut health" is not something a trial can test. Antibiotic-related diarrhea or IBS symptoms are.
- Find the strain in the research, then check the label for that exact strain. For IBS, the NHS suggests trying probiotics for a month. [15] It also says a type that helps one problem does not mean it will help another. [2]
- Ask a clinician first if you have a weakened immune system or serious illness, or are choosing for an infant. See probiotic safety.
- Do not delay care for a symptom that needs a diagnosis.
Where to go next
Prebiotic fibers are covered on prebiotic fibers and prebiotics vs probiotics. Goal-level advice is on probiotics, prebiotics and fermented foods for gut health.Frequently asked questions
Do probiotics work?
It depends on the strain and the condition. The AGA found insufficient evidence for most of the eight digestive situations it reviewed, with possible benefit for specific strains in a few. [1]
Which probiotic is best?
No single product is best for everyone. Evidence belongs to named strains and specific uses, and trial products are not always the ones on the shelf. [14][2]
Is yogurt a probiotic?
Not automatically. ISAPP says the terms "fermented food" and "probiotic" cannot be used interchangeably. [10]
Are probiotics regulated like medicines?
Generally not. In the US many are sold as dietary supplements without FDA approval before marketing, and in the UK the NHS says they are generally classed as food. [5][2]
Can I give a probiotic to a premature baby?
Not on your own. The FDA warned in 2023 that preterm infants given probiotics are at risk of invasive, potentially fatal infection. [4]
More details: evidence, limits and cautions
The details behind this page: cautions, limits of the evidence and where sources disagree. Open any note to read it.
How to read the evidence labels
Labels used on this page: promising but limited, mixed, insufficient evidence, marketing claim, and Proper evidence not found (we looked and could not find usable evidence). Every result below applies to the strains, products and people in those studies, not to every probiotic.
| Question | What the sources found | Label |
|---|---|---|
| Preventing diarrhea linked to antibiotics or C. difficile | A 2025 Cochrane review of 47 trials (15,260 people) pooled many different probiotics. It found lower rates of C. difficile-associated diarrhea (relative risk 0.50) and antibiotic-associated diarrhea (0.67), both low-certainty evidence. The authors noted probiotic-company affiliation or funding in 28 studies. [12] | Promising but limited |
| Irritable bowel syndrome (IBS) | NCCIH describes a 2018 review of 53 studies that could not identify which strains are most likely to help. [5] A 2026 review of single-strain, placebo-controlled trials found 10 strains studied: several showed efficacy, some did not, and one had conflicting results. [13] | Mixed, strain by strain |
| Eczema in infants and children | The NHS says there is no evidence probiotics can help treat eczema. [2] NCCIH describes a 2017 review of 13 studies that did not find consistent evidence of benefit. [5] | Insufficient evidence |
| Premature infants (necrotizing enterocolitis) | NCCIH describes a 2017 review of 23 studies in which probiotics helped prevent this gut illness in very-low-birth-weight infants. [5] The FDA's 2023 safety warning, above, applies to the same group. [4] | Mixed: benefit signal and documented harm; hospital decision only |
| General "gut health" in healthy adults | Proper evidence not found. We searched the AGA technical review (which covers eight digestive conditions, not healthy people), the NCCIH page and the NHS page. | Marketing claim |
Details are on do probiotics work? and probiotics for diarrhea, IBS, bloating and regularity.
Read this first: who should ask before using one
- Weakened immune system or an existing health condition. The NHS says to talk to a doctor before taking any probiotic supplement. [2] ISAPP, a scientific association, gives the same advice for serious medical conditions, especially those involving the gut or immune system. [3]
- Premature babies. In October 2023 the US Food and Drug Administration (FDA) warned that preterm infants given a probiotic are at risk of invasive, potentially fatal infection. The FDA also said it has not approved any probiotic product as a drug or biological product for infants of any age. [4] Any use in a neonatal unit is a decision for the hospital care team.
- Serious illness. NCCIH (US) says the risk of harm is greater in people with severe illness or a compromised immune system. [5]
- Do not delay care. NCCIH advises against using probiotics as a reason to postpone seeing a health care provider about any health problem. [5] In a US emergency, call 911.
- Medicines, pregnancy, children, older adults, kidney or liver conditions, and surgery. The sources we read for this page do not give specific advice for these groups. Ask a clinician or pharmacist, and see how medicine interactions work and probiotics and medicine interactions.
References
- Preidis GA, et al. AGA Technical Review on the Role of Probiotics in the Management of Gastrointestinal Disorders. Gastroenterology. 2020;159(2):708-738. Full text read via PubMed Central (PMC8018518). Read 2026-10-22.
- NHS (UK). Probiotics. Page last reviewed 24 November 2022. Read 2026-10-22.
- International Scientific Association for Probiotics and Prebiotics (ISAPP). Probiotics. Read 2026-10-22.
- US Food and Drug Administration. FDA Raises Concerns About Probiotic Products Sold for Use in Hospitalized Preterm Infants. News release, 26 October 2023. Read 2026-10-22.
- NCCIH (US National Center for Complementary and Integrative Health). Probiotics: Usefulness and Safety. Read 2026-10-22.
- Hill C, et al. The ISAPP consensus statement on the scope and appropriate use of the term probiotic. Nat Rev Gastroenterol Hepatol. 2014;11(8):506-14. Abstract read 2026-10-22.
- Gibson GR, et al. The ISAPP consensus statement on the definition and scope of prebiotics. Nat Rev Gastroenterol Hepatol. 2017;14(8):491-502. Abstract read 2026-10-22.
- Swanson KS, et al. The ISAPP consensus statement on the definition and scope of synbiotics. Nat Rev Gastroenterol Hepatol. 2020;17(11):687-701. Abstract read 2026-10-22.
- Salminen S, et al. The ISAPP consensus statement on the definition and scope of postbiotics. Nat Rev Gastroenterol Hepatol. 2021;18(9):649-667. Abstract read 2026-10-22; full text read via PubMed Central (PMC8387231).
- Marco ML, et al. The ISAPP consensus statement on fermented foods. Nat Rev Gastroenterol Hepatol. 2021;18(3):196-208. Abstract read 2026-10-22; full text read via PubMed Central (PMC7925329).
- International Scientific Association for Probiotics and Prebiotics (ISAPP). Prebiotics. Read 2026-10-22.
- Esmaeilinezhad Z, et al. Probiotics for the prevention of Clostridioides difficile-associated diarrhea in adults and children. Cochrane Database Syst Rev. 2025;9:CD006095. Abstract read 2026-10-22.
- Maslennikov R, et al. Strain-specific systematic review with meta-analysis of probiotics efficacy in the treatment of irritable bowel syndrome. J Clin Med. 2026;15(3):1152. Full text read via PubMed Central (PMC12898053). Read 2026-10-22.
- McFarland LV, Evans CT, Goldstein EJC. Strain-specificity and disease-specificity of probiotic efficacy: a systematic review and meta-analysis. Front Med. 2018;5:124. Full text read via PubMed Central (PMC5949321). Read 2026-10-22.
- NHS (UK). Diet, lifestyle and medicines for IBS. Page last reviewed 17 March 2025. Read 2026-10-22.
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.