Saccharomyces boulardii: The Yeast Probiotic Backed by Strong Trials

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Saccharomyces boulardii is unusual among probiotics: it is a yeast rather than a bacterium. That matters clinically because antibacterial antibiotics do not kill it, so it is one of the few probiotics that can be taken at the same time as a course of antibiotics. It also has some of the better randomised-trial evidence in the probiotic space.

Antibiotic-Associated Diarrhoea

The Szajewska & Kołodziej 2015 systematic review and meta-analysis in Alimentary Pharmacology & Therapeutics (PMID 26216624; DOI 10.1111/apt.13344) pooled 21 RCTs (n=4,780 children and adults). S. boulardii reduced the risk of antibiotic-associated diarrhoea from 18.7% to 8.5% (RR 0.47, 95% CI 0.38–0.57; NNT~10). The effect was consistent across paediatric (RR 0.43) and adult (RR 0.49) trials. Benefit is largest when supplementation starts on day 1 of antibiotics and continues a few days to a week beyond the antibiotic course.

Clostridioides difficile Recurrence

For preventing recurrence of C. difficile infection, S. boulardii added to standard therapy (vancomycin or fidaxomicin) reduced recurrence rates in some trials but results have been heterogeneous. The McFarland 2010 adult-focused meta-analysis (PMID 20458757; DOI 10.3748/wjg.v16.i18.2202) and the Szajewska 2015 review found a signal for C. difficile reduction in children (2 RCTs, n=579, RR 0.25) but not in pooled adult data (9 RCTs, RR 0.80, not significant). Current US IDSA / SHEA guidelines do not recommend probiotics for primary C. difficile prevention as a standard of care; some clinicians still use S. boulardii as an adjunct in selected cases.

Traveller’s Diarrhoea

Smaller trials suggest S. boulardii can modestly reduce the incidence of traveller’s diarrhoea when started a few days before travel and continued throughout the trip. The effect is on top of (not a replacement for) standard food and water hygiene. A typical preventive regimen is 250–500 mg twice daily.

Practical Notes

The specific strain matters: CNCM I-745 (sold as Florastor in the US and Ultra-Levure in much of Europe) is the strain with the largest evidence base. Typical dose is 250 mg one to two times daily. Unlike bacterial probiotics, it does not require refrigeration. Avoid in severely immunocompromised patients, neonates in intensive care, and patients with central venous catheters because of rare but serious case reports of Saccharomyces fungaemia in those settings.