Probiotics
Probiotics are live bacteria and yeasts taken for a health benefit. The best evidence is for specific strains: some lower the risk of diarrhea from antibiotics, and some ease IBS symptoms such as stomach pain. Results depend on the strain, so one product isn't interchangeable with another.
Probiotics are live microorganisms that, when taken in adequate amounts, give a health benefit. Products often contain bacteria or yeasts from groups such as Bifidobacterium, Saccharomyces, Streptococcus, Enterococcus, Escherichia, Bacillus and the Lactobacillaceae family. Doses are measured in colony-forming units (CFU), the number of live cells, and the best dose depends on the strain and product.
Research & evidence
Strong support Some support Weak or no supportColours show how well the sources back each point. The reason is next to each one.
Probiotics, including Saccharomyces boulardii, reduce the risk of antibiotic-associated diarrhea and modestly shorten its duration (by about a day) when it occurs. The strongest pediatric evidence (Cochrane, 2019) is specific to children; a separate adult-and-child analysis of S. boulardii (Szajewska & Kołodziej, 2015) found a similar risk reduction in adults.
Moderate evidenceSome support. Shown only in certain groups, uses, strains or doses
Study details
Guo et al. 2019 (Cochrane, ages 0-18): "The overall evidence suggests a moderate protective effect of probiotics for preventing AAD" (NNTB 9, 95% CI 7-13, overall; NNTB 6 in the high-dose ≥5 billion CFU/day subgroup). The authors add: "The benefit of high dose probiotics (e.g. Lactobacillus rhamnosus or Saccharomyces boulardii) needs to be confirmed by a large well-designed multi-centered randomized trial." Duration was reduced by about 0.91 days (low-certainty evidence). Szajewska & Kołodziej 2015 (21 RCTs, 4,780 participants, adults and children): S. boulardii reduced AAD risk from 18.7% to 8.5% overall (RR 0.47, NNT 10); in children, 20.9% to 8.8%; in adults, 17.4% to 8.2%. The Cochrane review was funded by the Hospital for Sick Kids Foundation, and its authors declare no conflicts.
Source: Guo Q, Goldenberg JZ, Humphrey C, El Dib R, Johnston BC. "Probiotics for the prevention of pediatric antibiotic-associated diarrhea." Cochrane Database Syst Rev. 2019 Apr 30. doi: 10.1002/14651858.CD004827.pub5; Szajewska H, Kołodziej M. "Systematic review with meta-analysis: Saccharomyces boulardii in the prevention of antibiotic-associated diarrhoea." Aliment Pharmacol Ther. 2015 Oct;42(7):793-801. View source
Certain probiotic strains — including Bifidobacterium longum 35624, Lactobacillus rhamnosus GG, Lactiplantibacillus plantarum 299v, and Bacillus coagulans MTCC5260 — have shown consistent, strain-specific benefit for IBS symptoms including abdominal pain, in meta-analyses of randomized trials. Some other strains showed no benefit, so "probiotics" generically is not interchangeable with these specific, named strains. Saccharomyces boulardii's evidence is mixed between reviews.
Moderate evidenceSome support. Shown only in certain groups, uses, strains or doses
Study details
Maslennikov et al. 2026 (32 articles, 10 strains, no conflicts declared): confirms efficacy for B. longum 35624, L. rhamnosus GG, L. plantarum 299v, S. cerevisiae CNCM I-3856, and B. coagulans Unique IS2 (MTCC 5260) on key IBS symptoms; a different B. coagulans strain (MTCC 5856) improved quality of life, not symptoms. It found no efficacy for E. coli Nissle 1917, L. gasseri BNR17, or L. casei Shirota. "Conflicting results were observed for Saccharomyces boulardii CNCM I-745." McFarland, Karakan & Karatas 2021 (42 RCTs, N=3,856) found S. boulardii CNCM I-745 significantly effective for abdominal pain (RR=1.5). Its lead author discloses being "on the Scientific Advisory Board of Bio-K+... and on the Biocodex Microbiome Board... and has received honoraria from Bio-K+ and Biocodex." The study itself states it was unfunded.
Source: Maslennikov R, Gosteeva E, Ananeva V, et al. "Strain-Specific Systematic Review with Meta-Analysis of Probiotics Efficacy in the Treatment of Irritable Bowel Syndrome." J Clin Med. 2026 Feb 2;15(3):1152; McFarland LV, Karakan T, Karatas A. "Strain-specific and outcome-specific efficacy of probiotics for the treatment of irritable bowel syndrome." EClinicalMedicine. 2021 Oct 18;41:101154. View source
The gut-brain axis, partly mediated by gut bacteria, is an active area of research. A 2026 umbrella review pooling 30 meta-analyses found consistent benefits of probiotic interventions for major depressive disorder (SMD -0.50); evidence for anxiety remains markedly inconsistent. The review didn't analyze results by strain or dose.
Moderate evidenceSome support. Results are mixed
Study details
Sá Filho et al. 2026 (umbrella review, 30 systematic reviews/meta-analyses, AMSTAR-2 assessed, no conflicts, no external funding): "Probiotic interventions demonstrated consistent benefits for MDD (SMD = -0.50 [95% CI: -0.58 to -0.42], p = 0.0001). However, findings for anxiety were markedly inconsistent... (SMD = -0.19 [95% CI: -0.28 to -0.10])." The review also states: "Methodological quality was predominantly moderate, low, or critically low in 76.6% of reviews" and "The present review did not perform strain-specific or dose-response analyses." Prebiotics showed a smaller effect for MDD (SMD -0.25) and an inconclusive effect for anxiety (SMD -0.07).
Source: Sá Filho AS, Souza TB, Martins JLR, et al. "'Attacking' the Gut-Brain Axis with Psychobiotics: An Umbrella Review of Depressive and Anxiety Symptoms." Pharmaceuticals (Basel). 2026 Jan 15;19(1):156. View source
Dosage
| Form | Amount |
|---|---|
| Any form | There are no formal recommendations for or against probiotics in healthy people, and the best dose depends on the strain and product. Many probiotic supplements contain 1 to 10 billion CFU per dose, and some contain 50 billion CFU or more. For preventing diarrhea from antibiotics, NIH notes that starting probiotics within 2 days of the first antibiotic dose works better than starting later. |
Side effects
Side effects are usually minor and short-lived, such as gas. If you're seriously ill or have a weakened immune system, talk to your doctor first: in rare cases, probiotics have been linked to blood infections.
Well documented Reported (case reports, anecdotal or one study) None reportedThe circle shows how well documented an effect is, not how serious it is. A rarely reported effect can still be serious, so always read ‘When to stop’.
| Effect | Frequency | When to stop |
|---|---|---|
| Gas and other mild, short-lived gut symptomsWell documentedListed plainly by NIH, MSK or LiverTox | Not stated; usually minor | Stop and talk to a doctor if it doesn't settle or gets worse. |
| Blood infections (bacteremia or fungemia)ReportedCase reports only | Rare; a few reported cases, mostly in people who were severely ill or immunocompromised | Stop and see a doctor right away if you feel seriously unwell. |
Safety & interactions
Safety
Probiotics are unlikely to cause harm in healthy people, and side effects are usually minor, such as gas. They have been linked to a few cases of bacteremia (bacteria in the blood), fungemia (fungi in the blood) and other infections that caused severe illness, mostly in people who were severely ill or immunocompromised. In 2023 the FDA warned that giving probiotics to preterm infants may cause infection or serious, potentially fatal disease.
Contraindications
Talk to your healthcare provider before taking probiotics if you have a weakened immune system or a central venous catheter. Babies who are underweight, have umbilical catheters or have low oxygen levels should not be given probiotics. In many reported cases of probiotic-linked fungemia, the patients were in intensive care, were receiving tube or IV nutrition, had a central venous catheter, or had received broad-spectrum antimicrobial treatment.
Drug interactions
NIH's probiotics fact sheet lists no drug interactions, and MSK's patient page doesn't list any either. Talk with your healthcare provider before taking probiotics if you take any medication.