Research synthesisModerate evidenceLarge effect4 studies · 3 beneficial · 1 neutral · 0 harmful
Across 4 studies, 3 reported beneficial effects of Lactobacillus plantarum 299v on irritable bowel syndrome symptoms, with effect sizes ranging from moderate to large; 1 study (a meta-analysis) found neutral results. The beneficial studies included a small randomized controlled trial (n=20) showing improvement in 95% of patients vs 15% with placebo (p<0.0001) and a larger observational study (n=221) showing significant symptom reduction. Effects are typically observed over 4–12 weeks, though median study duration was 28 days.
- Effective dose range: 1×10^10 CFU/day
- Studied populations: patients with IBS
Caveats: Available evidence is overwhelmingly positive — clinical literature in this area is subject to publication bias (null-result studies are less likely to be published or indexed). Evidence base is small (only 4 studies) — conclusions should be considered preliminary. The single meta-analysis (highest evidence score) did not demonstrate efficacy, which contradicts the smaller studies.
Generated Jun 17, 2026