Saccharomyces boulardii
CNCM I-745 · The Probiotic Yeast
Evidence grade 4 of 5, Strong. Read how the Atlas grades evidence.
27 human trials, about 90 studies, 5029 participants. The written rationale for this grade is still being prepared.
- FDA
- Pharmaceutical origin; Florastor
- WADA
- Not prohibited
Reference only. Not medical advice.
What can go wrong
In healthy, immunocompetent people the safety record is strong; the only common reported effects in trials were mild thirst and constipation. The real safety signal is fungemia (a bloodstream yeast infection, reported in the lab as Saccharomyces cerevisiae): documented case reports and regulator reviews describe it in critically ill, mechanically ventilated, central- or peripheral-venous-catheter, and immunocompromised patients, including fatal cases. Because it is a live yeast, spread has been reported even to nearby catheterized patients from airborne capsule contents during handling. European regulators (EMA 2017; national authorities such as Belgium's FAMHP, 2018) made these products contraindicated in seriously ill or immunosuppressed patients. Anyone who is critically ill, immunocompromised, has a central line, is pregnant, or is giving it to a child should not self-start this and should first consult a qualified healthcare provider, who can weigh it against their specific situation. This entry is informational and is not medical advice.
Saccharomyces boulardii is the most-studied probiotic yeast and, being a yeast rather than a bacterium, is intrinsically resistant to antibacterial antibiotics, so it can be taken alongside an antibiotic course when bacterial probiotics would be killed. Its strongest evidence is for antibiotic-associated diarrhea, where a meta-analysis of 27 adult trials pooled a relative risk of 0.47, and for traveler's diarrhea. Evidence for C. difficile recurrence is narrower (tied mainly to high-dose vancomycin), primary C. difficile prevention is not well supported, and IBS-D benefit is preliminary. It is sold OTC as Florastor by Biocodex under the strain designation CNCM I-745. The one real safety signal is fungemia in critically ill, central-line, or immunocompromised patients, where regulators have made it contraindicated; anyone in those groups should consult a qualified provider first.
A yeast, not a bacterium, so antibiotics cannot kill it. Take them together.
What people take it for
What it is taken for. Approval, where it exists, covers only some of these.
What people get wrong
Often saidIt is just another probiotic bacteria
ActuallyIt is a yeast, a strain of Saccharomyces cerevisiae, not a bacterium. That is precisely why antibacterial antibiotics do not kill it, which is its central and genuinely differentiated clinical advantage when taken during an antibiotic course (McFarland 2010 review).
Often saidIt reliably prevents C. difficile infection
ActuallyThe strong, meta-analytic evidence is for antibiotic-associated diarrhea (pooled RR 0.47), not C. difficile prevention. A 2023 systematic review found no significant benefit for primary or recurrent C. difficile infection, and the older recurrence signal appeared only alongside high-dose vancomycin in a single trial. The C. difficile claim is weaker than the AAD claim and should not be stated with the same confidence.
Often saidIt is safe for everyone because it is natural and OTC
ActuallyIt carries a documented fungemia risk in critically ill, central-line, and immunocompromised patients. European regulators (EMA 2017, national authorities 2018) formally contraindicated it in those groups after reviewing fatal bloodstream-infection cases, and advised care when handling capsules near catheterized patients. Broad safety in healthy adults does not extend to those populations.
Often saidS. boulardii is a completely separate species from baker's/brewer's yeast
ActuallyGenomic work (Khatri et al. 2017) shows it shares greater than 99% sequence identity with Saccharomyces cerevisiae and is best classified as a strain of that species. This is why fungemia isolates are typically identified in the lab as S. cerevisiae.
Who profits, and who pushes back
Travel and GI clinicians
Frequently recommended for travelers and antibiotic users; strong, consistent diarrhea evidence.
Florastor (Biocodex)
Sells this. Has a commercial interest in a product sold as this compound, as of July 2026.
Commercializes the pharmaceutical-origin strain OTC; genuine evidence behind the specific strain.
European Medicines Agency (CMDh)
The risk of using Saccharomyces boulardii-containing products in critically ill or immunocompromised patients outweighs the potential benefit due to the risk of fungaemia; use in these patients is contraindicated.
Critical-care and infectious-disease case authors (IDCases 2025, pancreatitis fungemia report)
Saccharomyces-based probiotics should be strictly avoided in patients with known or suspected immunocompromise or those with central lines.
2023 C. difficile prevention systematic review authors
The use of S. boulardii appears to have no benefit for preventing either primary or recurrent C. difficile infection in patients taking systemic antimicrobials.
Where the research comes from
| Place | Share of published research |
|---|---|
| France | 28% |
| USA | 26% |
| Europe | 24% |
| Other | 22% |
Research published from: France, Global.
How we got here
The strain traces to Southeast Asia, not a Western laboratory. In the early 1920s, French microbiologist Henri Boulard, working in Indochina during a cholera outbreak, observed that local people chewed the skins of lychee and mangosteen fruit, and brewed a tea from them, to calm severe diarrhea, and that those who did seemed protected. Boulard isolated the responsible yeast (commonly dated to 1923) from the fruit peels and named it after himself. The knowledge he drew on was the indigenous Indochinese fruit-skin remedy; the isolation formalized a practice that already existed in the region. Decades later the French pharmaceutical company Biocodex developed the reference strain, designated CNCM I-745 (also CBS 5926 / ATCC 74012), into a standardized product marketed in Europe as Enterol and in the United States over the counter as Florastor. Later genomic work reclassified it as a strain of Saccharomyces cerevisiae rather than a distinct species.
- 1920sHenri Boulard, in Indochina during a cholera outbreak, documents the indigenous lychee- and mangosteen-skin remedy for diarrhea
- 1923Boulard isolates the yeast from fruit peels and names it Saccharomyces boulardii
- 1950s-1960sBiocodex develops and commercializes the CNCM I-745 strain (Enterol) in France
- 1989Kollaritsch placebo-controlled trials in Austrian travelers show dose-dependent reduction of traveler's diarrhea
- 2000Surawicz/McFarland multisite RCT reports reduced C. difficile recurrence only in the high-dose vancomycin arm
- 2010McFarland systematic review of 27 adult RCTs pools RR 0.47 for antibiotic-associated diarrhea
- 2017Genomic analysis (Khatri et al.) confirms S. boulardii is a strain of S. cerevisiae, >99% identity
- 2017-2018EMA and national regulators contraindicate the product in critically ill and immunocompromised patients over fungemia risk
- 2023Systematic review finds no significant benefit of S. boulardii for primary or recurrent C. difficile prevention
At a glance
- Type
- Yeast (not bacteria)
- Strain
- CNCM I-745
- Key trait
- Antibiotic-resistant
- Brand
- Florastor
What this entry does not yet cover
What follows is unfinished work on our side, not a finding about Saccharomyces boulardii. A short entry is not a safe entry.
- Cost and market. We have not documented what it sells for or who sells it.
- Supply chain. We have not mapped where it is made.
- Who uses it. We have not documented the user groups.
- What could change. No forward-looking summary yet.
- How it compares. We have not set it against the alternatives.
- Children and teenagers. We have not checked what is known about younger people.
Sources
AAD pooled RR 0.47; 27 adult RCTs / 5,029 patients; indication-by-indication efficacy; Boulard discovery in Indochina
S. boulardii is a strain of S. cerevisiae (>99% identity); Boulard 1923 isolation from lychee/mangosteen in Indochina; CNCM I-745 / Florastor / Biocodex; fungemia risk in immunocompromised
Wikipedia, Saccharomyces boulardii (citing Khatri et al. 2017 genomics)
EMA 2017: risk in critically ill/immunocompromised outweighs benefit; contraindicated; ~half of fatal fungemia cases had a central venous catheter; product-handling warning near catheterized patients
Malaysia NPRA safety alert summarizing EMA CMDh conclusions on S. boulardii fungemia risk
Belgium FAMHP 2018: Enterol contraindicated in seriously ill or immunosuppressed patients; care handling near venous catheters; risk not excludable for food supplements
FAMHP (Belgian medicines agency), Flash VIG-news on Saccharomyces boulardii / Enterol
Case report: S. cerevisiae fungemia after S. boulardii in severe pancreatitis; risk factors (central line, broad-spectrum antibiotics, disrupted GI barrier, ICU); avoid in immunocompromised / central-line patients
Seven cases of S. boulardii fungemia in a 12-bed ICU, all mechanically ventilated patients with central venous catheters
Seven cases of fungemia with Saccharomyces boulardii in critically ill patients (case series)
C. difficile recurrence reduced (16.7% vs 50%, P=0.05) only in high-dose vancomycin arm; no effect at low-dose vancomycin or metronidazole
Surawicz/McFarland C. difficile recurrence RCT, as reported in McFarland 2010 meta-analysis
2023 systematic review: no significant benefit of S. boulardii for primary or recurrent C. difficile infection (pooled OR ~0.71, 95% CI 0.46-1.10, non-significant)
Kollaritsch traveler's-diarrhea RCTs: dose-dependent reduction (43% placebo vs 34% low dose vs 32% high dose) in Austrian travelers
IBS-D: S. boulardii improved cytokine profile and quality of life but not individual bowel symptoms (preliminary)
S. boulardii as adjunct improves H. pylori eradication rate and reduces therapy side effects (diarrhea); does not replace eradication regimen