Saccharomyces boulardii: The Yeast Probiotic Backed by Strong Trials
Saccharomyces boulardii is a yeast rather than a bacterium, which is why antibacterial antibiotics don’t kill it and it can be taken alongside an antibiotic course — and it has some of the strongest randomized-trial evidence of any probiotic. Its best-supported use is preventing antibiotic-associated diarrhoea: a 2015 meta-analysis of 21 trials found it cut the risk from 18.7 percent to 8.5 percent (about one case prevented for every ten people treated), with the biggest benefit when started on day one of antibiotics. The evidence is weaker and mixed for C. difficile recurrence (a signal in children but not in pooled adult data) and only modest for traveller’s diarrhoea. The strain with the data is CNCM I-745 (Florastor) at 250 mg once or twice daily, and it should be avoided in severely immunocompromised patients, ICU neonates, and anyone with a central venous catheter because of rare but serious Saccharomyces bloodstream infections.
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.
Sources
- Szajewska H, Kołodziej M. "Systematic review with meta-analysis: Saccharomyces boulardii in the prevention of antibiotic-associated diarrhoea." Alimentary Pharmacology & Therapeutics, 2015. PMID 26216624; DOI 10.1111/apt.13344.
- McFarland LV. "Systematic review and meta-analysis of Saccharomyces boulardii in adult patients." World Journal of Gastroenterology, 2010. PMID 20458757; DOI 10.3748/wjg.v16.i18.2202.
- Kelesidis T, Pothoulakis C. "Efficacy and safety of the probiotic Saccharomyces boulardii for the prevention and therapy of gastrointestinal disorders." Therapeutic Advances in Gastroenterology, 2012. PMID 22423260; DOI 10.1177/1756283X11428502.