Can Probiotics Cause Diarrhea?
Last updated: August 2026 · Reviewed by the FactoWiki Editorial Team for clarity and source accuracy
Yes, though it is usually mild and settles within a week or two. Loose stools when starting a probiotic are common as fermentation patterns shift. Persistent diarrhoea beyond a month is not adjustment and warrants stopping and looking at the cause.
Key takeaways
- Mild loose stools in the first 1–2 weeks are common and usually settle.
- The added prebiotic — inulin or FOS — is often the actual culprit.
- Persistent diarrhoea past a month needs investigating, not persevering.
Why it happens initially
Introducing new organisms shifts fermentation and gas production while the microbial balance resettles. Some existing populations decline. The result can be loose stools, urgency or gas for the first week or two. Starting at half a dose or every other day and building up usually avoids the worst of it.
Check the prebiotic first
Many probiotic products include inulin or FOS. Both are rapidly fermentable FODMAPs and both reliably produce gas and loose stools in sensitive people, particularly those with IBS. If a probiotic is causing trouble, read the label — the bacteria may be innocent. A product without added prebiotic often solves it entirely.
When it is not adjustment
Diarrhoea that persists beyond a month, or that is severe from the start, is a reason to stop. Consider SIBO, where adding bacteria can worsen symptoms. And diarrhoea with blood, weight loss, fever or night-time waking is not a probiotic question at all — that needs a doctor.
Key ingredients to understand
The ingredients most relevant to this question, with what the evidence actually supports:
- Probiotics (Lactobacillus & friends) — Probiotics are live microorganisms that can support digestion — but their effects are strain-specific, so the exact strains and dose matter more…
- Bacillus coagulans — B. coagulans is a spore-forming probiotic, which means it survives stomach acid, heat and years on a shelf without refrigeration. That practical…
- Saccharomyces boulardii — S. boulardii is a probiotic yeast, not a bacterium — which is why antibiotics do not kill it. It has some of the strongest evidence in the whole…
- Inulin (Prebiotic Fibre) — Inulin is a prebiotic soluble fibre (from chicory root) that feeds beneficial gut bacteria, supports regularity and calcium absorption, and adds…
Frequently asked questions
Should I push through it?
Give it two weeks at a reduced dose. If it is worse at a month, stop.
Which strains are gentlest?
Spore-forming Bacillus coagulans and Saccharomyces boulardii are generally better tolerated, and products without added inulin help.
Can probiotics treat diarrhoea?
Yes — that is one of their better-evidenced uses, particularly S. boulardii for antibiotic-associated diarrhoea. Same category, opposite direction.
Related on FactoWiki
- Gut & Digestive Health supplements — the full category
- All health answers
- Ingredient evidence guides
- Probiotics (Lactobacillus & friends) — ingredient guide
- Bacillus coagulans — ingredient guide
- How we review
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