Can Probiotics Cause Diarrhoea? When It Happens and How Long It Lasts

Live cultures can loosen the stool in the first few days, and that is the extent of it in the trial record: the pooled safety analyses list soft stools and wind among the early complaints, at rates no higher than placebo, and the one setting where diarrhoea has been measured properly, an antibiotic course, is the one where people taking live cultures had less of it, 8% against 19% in children's trials.^{1, 2} This page is one spoke of the Wellgard probiotics guide.

Evidence at a glance

• Trials: four pooled analyses of antibiotic-course trials (33 trials in 6,352 children; 42 in 11,305 adults; 36 in 9,312 adults; 37 safety-reporting trials in 11,911 people) and one Cochrane review of 82 stomach-bug trials in 12,127 people.^{1, 2, 3, 4, 5}
• Largest single trial: 2,981 UK hospital inpatients aged 65 and over, one multistrain capsule a day for 21 days.⁶
• Strongest measured outcome: diarrhoea during antibiotics, 8% on live cultures against 19% on placebo in the children's trials (relative risk 0.45).¹
• Not yet tested: when loose stools in the first days begin and end; no trial has timed them.

Sources checked on 16 October 2026.

Can probiotics cause diarrhoea?

Only mildly, and only for some people. In the 2025 Cochrane review of antibiotic-course trials, 37 trials in 11,911 people recorded an adverse event in 10.9% of those taking live cultures and 12.2% of controls, and the common events in both groups were abdominal cramping, nausea, fever, soft stools, flatulence and taste disturbance.⁴ Across 36 placebo-controlled adult trials in 9,312 people, the risk of any adverse event was the same on live cultures as on placebo (relative risk 1.00, 95% confidence interval 0.87 to 1.14).³ In the children's trials, 4% of those on live cultures and 6% of controls reported an event, the list running to rash, nausea, gas, flatulence, bloating and constipation.¹

Soft stools are on that list. Diarrhoea as an illness is not. A 2015 safety review cautions that trials report side effects inconsistently, so mild symptoms are probably under-counted.⁷

Why do I get diarrhoea after taking probiotics?

The usual explanation is fermentation: new organisms, and any prebiotic fibre in the same capsule, ferment in the large bowel and produce gas and water, and a looser stool for a day or two is what that feels like. No trial has traced the mechanism in people. It is also worth checking what else changed that week: the NHS lists medicines, a food intolerance, irritable bowel syndrome, anxiety and a stomach bug among the common causes, and a new capsule often arrives alongside a new antibiotic, which on its own gave diarrhoea to about one child in five on placebo.^{1, 8}

Can strong probiotics cause diarrhoea?

Not in the trials that tested high counts. In a three-week trial run by a culture manufacturer, 34 healthy adults took 100 billion CFU a day of a single Lactobacillus rhamnosus strain or placebo: gastrointestinal symptoms, stool frequency and stool consistency "were not influenced".⁹ In 100 adults with functional gut symptoms, 14 days at 17.2 billion or 1.8 billion CFU a day of a bifidobacterium strain produced no adverse events in any group.¹⁰ In the antibiotic-course trials the relationship ran the other way: children given 5 billion CFU a day or more had less diarrhoea than those on lower doses (relative risk 0.37 against 0.68).¹ The strength question in full, including what "too strong" feels like, is on how strong a probiotic should be.

How long does diarrhoea from probiotics last?

No trial has timed it, so any figure you read is an estimate. Two facts frame it. The NHS says diarrhoea from any common cause "usually stops within 5 to 7 days", and to call 111 if it lasts more than seven days, if there is blood in it, or if there are signs of dehydration such as dark, smelly pee.⁸ And in the pooled trials, early symptoms were mild and were as common on placebo as on live cultures.^{4, 3} The practical rule: a looser stool that settles within a few days is the pattern people report; one that is still going after a week or two is a reason to stop the capsule and speak to a pharmacist or GP.

What do the antibiotic trials show?

That live cultures taken through an antibiotic course cut the chance of diarrhoea, and that the product and the timing both matter.

• Children: across 33 trials in 6,352 children, diarrhoea during antibiotics occurred in 8% of those on live cultures and 19% of controls (relative risk 0.45; nine children taking them for one fewer case; moderate certainty).¹
• Adults: across 42 trials in 11,305 adults, the risk fell by 37% (relative risk 0.63, moderate certainty); a higher dose of the same product did better than a lower one, and trials with a low baseline risk showed no difference.²
• Timing: in 19 hospital studies of 6,261 adults, starting within two days of the first antibiotic dose cut Clostridioides difficile infection by two-thirds (relative risk 0.32); starting later did much less (0.70).¹¹
• The largest UK trial: 2,981 inpatients aged 65 and over given a multistrain capsule for 21 days, started up to a week into the course, had the same rate of diarrhoea as placebo (10.8% against 10.4%).⁶

When to start and how far from the tablet is on taking probiotics with antibiotics.

Can probiotics make diarrhoea worse?

Not in the trials that looked, though they did not shorten a stomach bug either. In the 2020 Cochrane review of 82 trials in 12,127 people with acute infectious diarrhoea, the two trials at low risk of bias found no difference in diarrhoea lasting 48 hours or more (relative risk 1.00, 1,770 people, moderate certainty), and the effect on duration was uncertain.⁵ In the largest, 971 children in US emergency departments given 10 billion CFU twice daily for five days had the same rate of moderate-to-severe illness as placebo (11.8% against 12.6%).¹² During a bug the NHS advice stands on its own: drink lots of fluids, and "do not have fruit juice or fizzy drinks".⁸

Can probiotics make IBS worse?

No pooled analysis records it. Across 55 IBS trials in more than 7,000 people, adverse events were not significantly more common on live cultures than on placebo, and no trial reports IBS worsening as a pooled outcome.¹³ The one documented route to feeling worse is unusual: a 2018 single-centre series of 30 people with gas, bloating and brain fog, all taking live cultures, found most had bacterial overgrowth in the small intestine; it is described on feeling worse before better.¹⁴ Which strains the IBS trials used is on probiotics for IBS.

What we don't know

• No trial has timed when first-days loose stools begin or end.
• No trial has compared a high-count product with a low-count one for side effects in healthy adults; the dose comparisons all come from antibiotic-course trials.
• No trial has tested whether taking the capsule with food, or at a different time of day, changes early symptoms.
• Adverse-event reporting in the older trials is inconsistent, so mild symptoms are probably under-recorded.⁷

Frequently asked questions

Should I stop taking probiotics if I get diarrhoea?

If it is mild and settling, the trial pattern says it passes. If it is still going after a week or two, or at any point there is blood, a fever or signs of dehydration, stop and follow the NHS advice to speak to a pharmacist or call 111.⁸

Do probiotics cause frequent bowel movements?

In adults with constipation, yes, by design: across 14 trials in 1,182 people, live cultures added 1.3 bowel movements a week.¹⁵ In adults with a normal bowel habit, frequency did not change in the 100 billion CFU trial.⁹ The detail is on do probiotics make you poop more.

Can too many probiotics cause diarrhoea?

No trial has found a count at which side effects rise. The 100 billion CFU tolerance trial is the highest daily dose we found tested in healthy adults, and it changed nothing in the bowel diary.⁹ Two capsules by mistake is well inside that. The full list of what the pooled trials record is on the side effects of too much probiotics.

Sources

1. Guo Q, et al. Probiotics for the prevention of pediatric antibiotic-associated diarrhea. Cochrane Database of Systematic Reviews, 2019. Pooled analysis of 33 randomised trials, 6,352 children. PubMed 31039287.
2. Goodman C, et al. Probiotics for the prevention of antibiotic-associated diarrhoea: a systematic review and meta-analysis. BMJ Open, 2021. Pooled analysis of 42 randomised trials, 11,305 adults. PubMed 34385227.
3. Liao W, et al. Probiotics for the Prevention of Antibiotic-associated Diarrhea in Adults: A Meta-Analysis of Randomized Placebo-Controlled Trials. Journal of Clinical Gastroenterology, 2021. Pooled analysis of 36 randomised trials, 9,312 adults. PubMed 33234881.
4. Esmaeilinezhad Z, et al. Probiotics for the prevention of Clostridioides difficile-associated diarrhea in adults and children. Cochrane Database of Systematic Reviews, 2025. Cochrane review; adverse events from 37 randomised trials, 11,911 people. PubMed 40931979.
5. Collinson S, et al. Probiotics for treating acute infectious diarrhoea. Cochrane Database of Systematic Reviews, 2020. Cochrane review, 82 randomised trials, 12,127 people. PubMed 33295643.
6. Allen SJ, et al. Lactobacilli and bifidobacteria in the prevention of antibiotic-associated diarrhoea and Clostridium difficile diarrhoea in older inpatients (PLACIDE). The Lancet, 2013. Randomised double-blind trial, 2,981 UK inpatients aged 65 and over, 21 days; publicly funded. PubMed 23932219.
7. Doron S, Snydman DR. Risk and safety of probiotics. Clinical Infectious Diseases, 2015. Review. PubMed 25922398.
8. NHS. Diarrhoea and vomiting. nhs.uk. Page last reviewed 21 December 2023.
9. Wind RD, et al. British Journal of Nutrition, 2010 (tolerance and safety of a Lactobacillus rhamnosus strain in healthy volunteers). Randomised double-blind trial, 34 healthy adults, 3 weeks; run by a culture manufacturer. PubMed 20691131.
10. Waller PA, et al. Scandinavian Journal of Gastroenterology, 2011 (dose-response of a Bifidobacterium lactis strain on whole-gut transit time). Randomised trial, 100 adults, 14 days; strain supplied by its manufacturer, funding not checked. PubMed 21663486.
11. Shen NT, et al. Gastroenterology, 2017 (timing of live cultures and Clostridium difficile in hospitalised adults; systematic review with meta-regression). Pooled analysis of 19 randomised trials, 6,261 adults. PubMed 28192108.
12. Schnadower D, et al. New England Journal of Medicine, 2018 (a Lactobacillus rhamnosus strain against placebo for acute gastroenteritis in children). Randomised double-blind trial, 971 children, 5 days. PubMed 30462938.
13. Goodoory VC, et al. Efficacy of Probiotics in Irritable Bowel Syndrome: Systematic Review and Meta-analysis. Gastroenterology, 2023. Pooled analysis of 82 randomised trials, 10,332 people; adverse events from 55 trials. PubMed 37541528.
14. Rao SSC, et al. Clinical and Translational Gastroenterology, 2018. Single-centre case series, 30 patients and 8 comparators; funding not checked. PubMed 29915215.
15. Dimidi E, et al. The effect of probiotics on functional constipation in adults: a systematic review and meta-analysis of randomized controlled trials. American Journal of Clinical Nutrition, 2014. Pooled analysis of 14 randomised trials, 1,182 adults. PubMed 25099542.

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