Saccharomyces boulardii: The Only Probiotic Yeast (2026)
Updated April 2026 · Strain: CNCM I-745 (Florastor) · Sources: Cochrane, Biocodex, NIH ODS
What It Is
Saccharomyces boulardii is a non-pathogenic, thermophilic yeast first isolated from lychee and mangosteen fruit in Indochina by French microbiologist Henri Boulard in 1923. The most clinically validated strain is S. boulardii CNCM I-745, marketed globally as Florastor (Biocodex).
Its status as a yeast (not a bacterium) confers a unique clinical advantage: antibiotics do not kill it. Patients taking antibiotics can simultaneously take S. boulardii without reducing its efficacy - the yeast survives the antibiotic course unharmed, providing gut protection throughout. No bacterial probiotic shares this property.
S. boulardii has a distinct mechanism of action compared to Lactobacillus species. Rather than colonising the gut persistently, it acts transiently and primarily through:
- Secreting a protease that cleaves C. difficile toxins A and B
- Secreting a phosphatase that inactivates E. coli LT (heat-labile toxin) and V. cholerae toxin
- Competing with pathogenic bacteria for binding sites on intestinal epithelial cells
- Stimulating IgA secretion and mucosal immunity
- Reducing intestinal permeability
What the Evidence Says
Antibiotic-Associated Diarrhoea (AAD) Prevention
Strong EvidenceThe most clinically important application. A 2015 Cochrane review (Goldenberg et al.) of 21 RCTs found S. boulardii reduces AAD risk by 54% (RR 0.46, 95% CI 0.35-0.60) in adults. The NNT (number needed to treat to prevent one case) is approximately 8. Recommendation: start 250-500 mg S. boulardii CNCM I-745 twice daily on the first day of antibiotic therapy; continue for 2-4 weeks after the course ends.
Cite: Goldenberg et al., Cochrane Database Syst Rev 2015; McFarland, World J Gastroenterol 2010
C. difficile Infection Prevention
Strong EvidenceS. boulardii is the most evidence-backed probiotic for C. diff prevention. The mechanism is direct: the yeast secretes a 54 kDa protease that specifically cleaves C. difficile toxin A receptor binding domain and inactivates toxin B. Clinical trial evidence includes Surawicz et al. (1989) and multiple subsequent RCTs. The Cleveland Clinic and many hospital antimicrobial stewardship programmes recommend S. boulardii as a standard adjunct to C. diff antibiotic therapy.
Cite: Surawicz et al., Gastroenterology 1989; Lau & Chamberlain, World J Gastroenterol 2016
Traveller's Diarrhoea Prevention
Strong EvidenceMultiple RCTs support S. boulardii CNCM I-745 for traveller's diarrhoea prophylaxis. Start 3-5 days before travel, continue throughout the trip and for 5 days after return. Dose: 250-500 mg twice daily. The effect is primarily against enterotoxigenic E. coli and Vibrio cholerae via the secreted phosphatase mechanism. Meta-analysis NNT for prevention: approximately 10.
Cite: McFarland, World J Gastroenterol 2007; Kollaritsch et al., Digestion 1993
Acute Diarrhoea (Any Cause)
Strong EvidenceThe WHO Oral Rehydration Solution (ORS) + zinc is the first-line intervention for acute diarrhoea. Adding S. boulardii to this standard care protocol reduces duration of diarrhoea by approximately 1 day compared to ORS alone. ESPGHAN considers S. boulardii (alongside LGG) as having sufficient evidence to recommend for acute gastroenteritis in children.
Cite: Szajewska et al., J Pediatr Gastroenterol Nutr 2020
IBS (General)
Limited EvidenceS. boulardii's mechanism is primarily anti-infective and barrier-restoring. For non-infectious IBS, the evidence is weaker than for Lactobacillus/Bifidobacterium strains. Some evidence for IBS-D (diarrhoea-type) where an infectious trigger may be present. Not a first-line choice for IBS per the 2024 network meta-analysis.
Cite: Ford et al., Am J Gastroenterol 2024
Dosing
Standard dose: 250-500 mg twice daily (Florastor capsules are 250 mg S. boulardii CNCM I-745 with 5 billion cells per capsule). The dose can be scaled to 500 mg three times daily for high-risk AAD prevention in immunocompromised patients.
S. boulardii is a true shelf-stable probiotic. It does not require refrigeration. The organism's temperature optimum is 37C (body temperature), which confers inherent stability at room temperature. This makes it ideal for travel use.
Unlike bacterial probiotics, S. boulardii can be taken at the same time as antibiotics without interaction. No separation interval is required.
Safety
S. boulardii is generally very safe. The most significant safety concern is fungaemia (Saccharomyces in the bloodstream), which has been documented primarily in patients with central venous catheters. Case reports suggest aerosolisation of S. boulardii powder near CVC insertion sites can lead to bloodstream infection - this is a real and serious risk. Do not administer S. boulardii in any patient with a CVC. Similarly, severely immunocompromised patients (bone marrow transplant, advanced HIV, etc.) should discuss S. boulardii use with their clinician before starting.
For healthy adults, children, pregnant women (limited data but no signal of harm at standard doses), and the elderly without immunocompromise, S. boulardii has an excellent safety record with no serious adverse events attributable to the strain in clinical trials.
Products
Affiliate disclosure: links below may be affiliate links.
Florastor Daily Probiotic (250 mg CNCM I-745, 2x/day)
Strong EvidenceThe reference product. Contains the exact strain and dose used in clinical trials. Available in capsules and sachets. The clinical gold standard. ~$30-40 for 30-day supply.
Florastor Kids (250 mg CNCM I-745, sachet)
Strong EvidencePowder sachet format for children. Same strain and dose. Mixes into food or drink. Appropriate from infancy (always consult paediatrician for use in infants under 1 year).
Jarrow Saccharomyces boulardii + MOS (5 billion/capsule)
Emerging EvidenceContains S. boulardii with mannan-oligosaccharides (MOS). Not the CNCM I-745 strain designation. Evidence is less direct than Florastor.