Research review
Probiotics: what the research does and doesn't show
Effects are strain- and purpose-specific. Pooled trials show signals for antibiotic-related diarrhea, chronic constipation and colds, with low confidence and mixed products.
The evidence does not support probiotics as a general fix for digestion, immunity, weight or mood. Trials test named strains for specific purposes, and a review of 249 trials found efficacy was both strain-specific and purpose-specific. Where pooled trials show a signal (antibiotic-related diarrhea, chronic constipation, colds), it comes with low confidence and mixed products.
What the evidence doesn't support
- "Probiotics" as one product with one effect. Strain, dose and purpose decide the result, and many products have no trial of their own.
- A change in gut bacteria in healthy adults. Seven placebo-controlled trials found no effect on the diversity, richness or evenness of stool bacteria.
- A fix for bloating. In 13 trials of Bifidobacterium animalis subsp. lactis in healthy adults, abdominal pain and bloating did not improve.
- Proof that they are risk-free. Short-term trials showed no excess of side effects, but safety reporting was often poor and long-term effects are largely unknown.
What the trials found
Diarrhea
A pooled analysis of 63 randomized trials (11,811 people, adults and children, taking antibiotics) found diarrhea after antibiotics was less common with a probiotic than without (relative risk 0.58, 95% CI 0.50 to 0.68; about 13 people treated for one to be spared). Strains were poorly documented and results varied widely. It does not cover other types of diarrhea.
Constipation
A review of 30 randomized trials in adults with chronic constipation found 57% responded to probiotics against 44% on control (relative risk 1.28), with stool frequency rising for Bifidobacterium lactis but not for mixtures of probiotics, and no change in stool consistency. Confidence was limited by heterogeneity and risk of bias.
Bloating and stomach pain
In 82 trials in adults with irritable bowel syndrome, certainty was low for a benefit of Saccharomyces cerevisiae I-3856 and Bifidobacterium strains on abdominal pain, and very low for combination probiotics and Bacillus strains on bloating. In healthy adults, 13 trials of B. animalis subsp. lactis found stool frequency rose slightly (SMD 0.26) but pain and bloating did not improve.
Colds and immunity
A Cochrane review of 12 randomized trials (3,720 children, adults and older people) found fewer people had at least one upper respiratory infection with probiotics than placebo (OR 0.53) and episodes were about 1.9 days shorter, but the quality of evidence was mostly low.
Weight
Pooled differences were small: a mean difference of -0.52 in body weight in a review of 12 trials in overweight adults, with larger effects for single strains at 10 billion CFU a day or more. See probiotics and weight.
Mood
In 12 trials of healthy working adults, probiotics gave a modest drop in self-reported depression, anxiety and stress and in cortisol; evidence on sleep was too limited. See probiotics and stress.
Safety
The AHRQ report of 622 studies found no significant increase in overall (RR 1.00) or serious adverse events (RR 1.06) in short-term randomized trials, with case reports of bloodstream infection and long-term effects largely unknown. A review of 384 trials found that 28% of the trials gave no harms data. See probiotics side effects.
Questions people ask
Can probiotics cause diarrhea, stomach pain or bloating? Why do they, and when?
Randomized trials found no significant excess of adverse events with probiotics versus control, with gastrointestinal events the ones mainly reported. People do report gas, loose stools or discomfort, particularly when starting. We have no verified trial that explains why, so we do not. If symptoms last, stop and ask a doctor or pharmacist.
Can probiotics cause constipation?
We found no verified trial showing that. In trials in constipation, certain strains increased stool frequency.
Will probiotics help with diarrhea, which are best, and will they stop it?
The antibiotic-related result above is the verified one, with poorly documented strains, so we cannot name the best. Diarrhea that lasts more than a few days, or comes with blood, fever or dehydration, needs a doctor.
Does probiotics help bloating, or constipation?
See the results above: constipation, a signal for one species; bloating, mostly very low-certainty or no improvement. Read probiotics and gut conditions for diagnosed disease.
Will probiotics help your immune system?
The Cochrane result on colds is the verified one, with low-quality evidence. No wider immune claim is made here.
Will probiotics make you gain weight?
In overweight adults, pooled trials found small decreases, not gains. We did not verify trials in other groups.
What about pregnancy, breastfeeding, kids, teens, dogs and cats?
Those are separate questions that this page does not answer, and we give no guidance on them. Ask your doctor or pharmacist, or a vet for a pet. See who should not take probiotics.
What do probiotics do to your mood and hormones?
The mood result is above. For hormones, a small trial of one L. brevis formula in menopausal women found blood estrogen was maintained versus placebo; see probiotics and menopause.
Basics: what are probiotics and probiotics benefits.
When to talk to a doctor
A supplement is not a treatment for any condition. Persistent diarrhea or constipation, blood in stool, severe pain, fever, vomiting or unexplained weight change need a medical assessment. If you are pregnant or nursing, have a weakened immune system or take prescription medicine, ask a doctor or pharmacist before starting any probiotic.
Sources
- Choosing an appropriate probiotic product for your patient: An evidence-based practical guide A review of 249 trials found probiotic effects depend on the exact strain and the purpose, and many products have no trial of their own.
- Alterations in fecal microbiota composition by probiotic supplementation in healthy adults: a systematic review of randomized controlled trials A review of 7 placebo-controlled trials in healthy adults found probiotics did not change the diversity of stool bacteria.
- Effects of Bifidobacterium animalis subspecies lactis supplementation on gastrointestinal symptoms: systematic review with meta-analysis In healthy adults, B. animalis subsp. lactis in 13 trials raised stool frequency slightly but did not improve abdominal pain or bloating.
- Probiotics for the prevention and treatment of antibiotic-associated diarrhea: a systematic review and meta-analysis A pooled analysis of 63 trials found diarrhea after antibiotics was less common with a probiotic than without, though strains were poorly documented and results varied widely.
- Probiotics and synbiotics in chronic constipation in adults: A systematic review and meta-analysis of randomized controlled trials A review of 30 trials in adults with chronic constipation found probiotics increased stool frequency, driven by one species, not by mixtures, with low confidence.
- Efficacy of Probiotics in Irritable Bowel Syndrome: Systematic Review and Meta-analysis A review of 82 trials in adults with IBS found some strains may help, but confidence in the evidence was low to very low almost everywhere.
- Probiotics for preventing acute upper respiratory tract infections A Cochrane review of 12 trials found fewer people had a cold-type infection with probiotics than placebo, but the evidence was low quality.
- Efficacy of probiotic supplementation for body weight management in overweight and obese adults: a meta-analysis of randomized controlled trials predominantly from East Asia A review of 12 trials, mostly in East Asia, found probiotics were associated with small reductions in body weight and BMI in overweight adults, mostly with single strains at high doses.
- Probiotic intake and mental health in healthy working adults: a systematic review and meta-analysis of randomized controlled trials A 2026 review of 12 trials in working adults found a modest drop in self-reported stress symptoms and cortisol, but no change in CRP.
- Safety of probiotics used to reduce risk and prevent or treat disease Short-term trials found no more side effects with probiotics than with control, but safety reporting was poor and long-term effects are largely unknown.
- Harms Reporting in Randomized Controlled Trials of Interventions Aimed at Modifying Microbiota: A Systematic Review A review of 384 trials found harms were often poorly reported, so safety cannot simply be assumed.
- Supplementation with a Probiotic Formula Having β-Glucuronidase Activity Modulates Serum Estrogen Levels in Healthy Peri- and Postmenopausal Women In a small 12-week trial a specific L. brevis formula kept blood estrogen levels from falling in women going through menopause, compared with placebo; symptoms were not the reported outcome.
*These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.