Saccharomyces boulardii
Saccharomyces boulardii is a probiotic yeast (not a bacterium) with the strongest specific clinical evidence base of any probiotic for preventing antibiotic-associated diarrhoea and for shortening acute infectious diarrhoea in children. It is unaffected by antibacterial antibiotics — a meaningful practical advantage over bacterial probiotics during an antibiotic course.
Camden Medicals editorial · Last reviewed 27 April 2026 · Next review April 2027
- Cross-checked against
- NHS
- NICE
- BNF
- EFSA
- FSA
Camden's editorial team independently graded each health claim below on the strength of the published evidence — see the grade beside every condition.
- Class
- Probiotic
- Typical daily dose
- Most published trials use 250-500 mg lyophilised yeast per dose, taken once or twice daily — equivalent to roughly 5 × 10⁹ CFU/day total.
- Top use evidence
- Moderate
On this page
What it is
Saccharomyces boulardii is a non-pathogenic yeast first isolated from lychee and mangosteen fruit by French scientist Henri Boulard in Indochina in the 1920s, after he observed locals using the fruit's skin to make a tea against cholera. Genetically it is closely related to brewers'' yeast (Saccharomyces cerevisiae) — modern molecular studies classify it as a strain or sub-species of S. cerevisiae rather than a wholly distinct species — but it has distinct properties: it grows optimally at 37°C (body temperature, where most S. cerevisiae strains are less efficient), and it is more acid- and bile-tolerant.
Commercial supplement strains include S. boulardii CNCM I-745 (the Biocodex strain, sold as Florastor in some markets and the most- trialled strain), and S. boulardii var. boulardii. As with bacterial probiotics, the trial evidence is for specific named strains at specific doses — most often 250 to 500 mg of lyophilised yeast per capsule, equivalent to roughly 5 × 10⁹ CFU.
At a glance
- A probiotic YEAST, not a bacterium — closely related to brewers' yeast (Saccharomyces cerevisiae) but with distinct properties.
- The strongest specific clinical evidence of any probiotic for preventing antibiotic-associated diarrhoea (AAD) and for paediatric acute infectious diarrhoea.
- Not destroyed by antibacterial antibiotics — can be co-administered without spacing.
- No GB-authorised health claim.
- Caution in immunocompromised people, critically ill patients, and central-venous-catheter users — fungaemia is a documented rare risk.
What people use it for
Adults starting a course of antibiotics, particularly broad-spectrum antibiotics or with a history of AAD
Multiple meta-analyses including Cochrane CD004827 and a dedicated McFarland 2010 meta-analysis support S. boulardii for reducing the risk of antibiotic-associated diarrhoea. Number needed (NNT) ~1 in 10 to 1 in 13. [1]
Some evidenceModerateAdults at risk of Clostridioides difficile-associated diarrhoea (CDAD)
Cochrane CD006095 supports probiotics including S. boulardii for CDAD prevention in patients receiving antibiotics. Evidence is moderate-quality and best when started early in the antibiotic course. [2]
Some evidenceModerateChildren with acute infectious diarrhoea (rotavirus and other viral causes)
Multiple paediatric trials and meta-analyses support S. boulardii for shortening duration of acute diarrhoea by approximately one day. ESPGHAN (European Society for Paediatric Gastroenterology, Hepatology and Nutrition) recommends it as one option alongside oral rehydration. Use for children should be discussed with a GP. [3,5,6]
Some evidenceModerateTravellers
S. boulardii has been studied for traveller's diarrhoea prevention with mixed results — some trials show modest benefit, others do not. Not a substitute for food/water hygiene practices.
Some evidenceLimited
How it works
S. boulardii is not believed to colonise the human gut permanently — counts in stool fall away within 3 to 5 days of stopping supplementation. While present, the yeast appears to act through multiple mechanisms: secretion of an enzyme that degrades Clostridioides difficile toxins A and B, competitive inhibition of enteric pathogen binding, modulation of intestinal-barrier protein expression, and local immune signalling. The clinical effects in AAD prevention and infectious diarrhoea are well-replicated; the underlying mechanism is multi-factorial and still being characterised.
Common myths
Myth"S. boulardii will permanently colonise my gut"
RealityIt will not. Stool counts fall away within 3 to 5 days of stopping supplementation. The clinical effect is "as long as you take it"; once you stop, the yeast clears.
Myth"S. boulardii is the same as brewers' yeast"
RealityGenetically very closely related — modern molecular taxonomy classifies S. boulardii as a strain of Saccharomyces cerevisiae — but the supplement-grade S. boulardii has distinct physiological properties (37°C growth optimum, acid + bile tolerance) and a specific clinical evidence base that brewers'' yeast does not share.
Myth"Probiotic yeast suppresses bacterial probiotics if taken together"
RealityNo published evidence to support this. Multi-strain formulations commonly include S. boulardii alongside bacterial probiotics; published trials of mixed formulations have not reported a suppressive interaction.
Common online questions
Synthesised from the questions UK shoppers most often ask online about Saccharomyces boulardii. Each answer is editorial and links to its evidence in the Sources list below.
Can I take S. boulardii alongside my antibiotic?
Yes — and this is one of its main practical advantages. Because S. boulardii is a yeast, antibacterial antibiotics do not destroy it (unlike Lactobacillus or Bifidobacterium, which can be reduced by simultaneous antibiotic dosing). No spacing is needed. The Cochrane evidence is strongest when probiotic is started within 2 days of antibiotic start. [1]
Should I give my child S. boulardii for diarrhoea?
Talk to your GP first. ESPGHAN 2014 supports it as one option for paediatric acute infectious diarrhoea alongside oral rehydration, but persistent or severe diarrhoea in a child needs medical assessment — not self-treatment with a probiotic. Dehydration is the main risk in paediatric diarrhoea, and oral rehydration solution (e.g. Dioralyte) is the priority intervention. [3]
How long should I take S. boulardii after antibiotics?
Most prevention trials run S. boulardii for the duration of the antibiotic course plus a few days afterwards (typically until 7-14 days post-antibiotic). For an established C. difficile infection, S. boulardii has been studied as adjunctive therapy but is not a replacement for proper antibiotic treatment of CDI — see your GP or hospital team. [2]
Is S. boulardii safe in pregnancy?
S. boulardii has been used in some research contexts in pregnancy. For routine use during pregnancy, talk to your midwife or pharmacist. As with all probiotics, immunocompromise or critical illness changes the risk-benefit calculation.
Vegans and S. boulardii — is it suitable?
Yes. S. boulardii is a yeast, not an animal product, and the manufacturing process does not use animal-derived ingredients in most commercial formulations. Check the capsule shell — gelatin capsules are common, but HPMC (vegan) alternatives exist for the same active.
Camden guides citing Saccharomyces boulardii
Editorial pieces from the Camden blog that reference Saccharomyces boulardii. Each guide cites the evidence it draws on.
🔬 Camden’s evidence review
The research Camden reviewed, graded on its strength. This is our own appraisal — it sits beneath the official guidance above, never above it.
Antibiotic-associated diarrhoea (AAD) prevention
ModerateEvidencemoderateCochrane CD004827.pub5 (Guo 2019) plus the dedicated McFarland 2010 meta-analysis support S. boulardii for AAD prevention. Effect is dose-responsive up to ~5 × 10⁹ CFU/day, and strongest when started within 2 days of antibiotic start. [1]
Clostridioides difficile-associated diarrhoea prevention
ModerateEvidencemoderateCochrane CD006095 supports probiotics for CDAD prevention in patients receiving antibiotics; S. boulardii is among the most-cited single agents. Number needed (NNT) in the order of 1 in 26 to 1 in 30 in inpatient settings. [2]
Paediatric acute infectious diarrhoea
ModerateEvidencemoderateMultiple meta-analyses support S. boulardii for shortening acute paediatric diarrhoea by ~1 day. ESPGHAN 2014 working group recommendation supports use as one option alongside oral rehydration. [3,5,6]
IBS, IBD maintenance, and traveller's diarrhoea
LimitedEvidencelimitedSmaller trials with mixed results across these indications. Not currently first-line in any UK guideline for these uses.
Safety
Saccharomyces boulardii has a long safety record at typical food-supplement doses for most healthy adults. CRITICAL caution in immunocompromised, critically ill, and central-venous-catheter patients — documented rare reports of fungaemia in these specific populations.
Talk to your pharmacist or GP first if you:
- You are immunocompromised, on chemotherapy, or have a low CD4 HIV count.
- You have a central venous catheter (CVC) — multiple case reports of S. boulardii fungaemia traced to environmental contamination of the CVC by capsule contents during opening. Do NOT open S. boulardii capsules in a room where a CVC patient is.
- You are critically ill or in intensive care.
- You have inflammatory bowel disease in active flare.
- You are pregnant or breastfeeding.
- You are giving to a child for diarrhoea — see your GP.
Common side effects: Generally very well tolerated. Mild bloating or wind in the first few days; settles with continued use.
Pregnancy and breastfeeding
Used in some research-context trials in pregnancy. Talk to your midwife or pharmacist before routine use.
Talk to your pharmacist or GP.
Talk to your GP, midwife, or pharmacist before starting any supplement during pregnancy or breastfeeding.
More clinical detail (for clinicians and informed readers)
Contraindications
- Severe immunocompromise.
- Critical illness with central venous catheter.
- Known yeast allergy.
Drug interactions
- Antifungal medications (e.g. fluconazole, nystatin) — will reduce or eliminate S. boulardii; do not co-administer.
- Antibacterial antibiotics — NO interaction; can be co-administered without spacing (a key practical advantage).
This is not an exhaustive list. Always tell your prescriber and pharmacist about every supplement you take.
Common side effects
- Mild bloating or flatulence in the first 1-2 weeks.
Rare side effects
- Fungaemia — case reports almost exclusively in immunocompromised or critically-ill patients with CVCs. Environmental contamination during capsule opening is the most-cited transmission route.
- Allergic reactions in individuals with yeast allergy (rare).
How to take it
- Typical supplemental range
- Most published trials use 250-500 mg lyophilised yeast per dose, taken once or twice daily — equivalent to roughly 5 × 10⁹ CFU/day total.
- Timing
- No spacing needed from antibiotic doses (yeast is unaffected by antibacterial antibiotics).
How to spot quality
Look for
- Named strain identifier on the label (e.g. "S. boulardii CNCM I-745").
- CFU or mg of lyophilised yeast per dose, AT END OF SHELF LIFE.
- GMP-certified manufacture.
Red flags
- Generic "S. boulardii" without strain identifier.
- Marketing claims about IBS treatment, immunity, or weight loss beyond authorised claims.
Commonly combined with
Other ingredients that share a biological pathway, cofactor relationship, or evidence-backed protocol with this one. Mechanisms below cite primary physiology — not folk pairing. Doses are literature ranges, not recommendations. Talk to your pharmacist or GP before changing your stack, especially if you take prescription medication.
Lactobacillus rhamnosus
Moderate evidenceS. boulardii and L. rhamnosus are the two probiotic species with the strongest individual evidence for antibiotic-associated diarrhoea prevention; they are commonly co-formulated.
Antibiotic-associated diarrhoea (AAD) is driven by gut-microbiome disruption. Saccharomyces boulardii (a non-pathogenic yeast) acts independently of antibiotics (no bacterial-antibiotic-killing effect), produces protease that cleaves Clostridioides difficile toxin A, and competes with pathogens for adhesion sites. Lactobacillus rhamnosus GG (a bacterial probiotic) competes for adhesion sites, produces antimicrobial peptides and short-chain fatty acids, and modulates gut immune signalling. The species are non-redundant in mechanism and can be combined.
Evidence: Goldenberg 2017 Cochrane review (Cochrane Database Syst Rev 12:CD006095) of 39 RCTs with 9,955 participants found probiotics reduce C. difficile-associated diarrhoea by 60% (NNT 42 overall, NNT 12 in high-baseline-risk patients). S. boulardii and L. rhamnosus GG are the species with the strongest individual evidence; combination products are common, although the Cochrane analysis did not find a clear advantage of multi-strain over single-strain formulations. [7,8,9]
Doses studied: S. boulardii: 250-500 mg twice daily for the duration of the antibiotic course plus a few days. L. rhamnosus GG: 10-20 billion CFU daily.
Bifidobacterium animalis subsp. lactis (BB-12, HN019, DN-173 010)
Moderate evidenceAAD prevention cluster — both have AAD-prevention evidence via different mechanisms (live yeast vs lactic-acid bacterium).
S. boulardii antibiotic-resistance (yeast) + B. animalis lactis Bifidobacterium delivery — mechanism-complementary across two organism domains. Both are AAD-prevention evidence-supported.
Evidence: Each has individual AAD-prevention trial body; combination not directly trialled head-to-head. [10]
Doses studied: S. boulardii 250-1000 mg/day + B. animalis BB-12 10⁹-10¹⁰ CFU/day during antibiotic course.
Echinacea (Echinacea purpurea / E. angustifolia / E. pallida)
Insufficient evidenceGut-immune axis pairing — echinacea innate-immune modulation alongside S. boulardii probiotic gut-mucosal effect for broader immune-system framing.
S. boulardii is a yeast probiotic with documented gut-immune-axis effect — modulates intestinal IgA secretion, dendritic-cell function in the gut-associated lymphoid tissue (GALT), and brush-border enzyme expression. Echinacea works on systemic innate-immune cells via alkamides and polysaccharides.
The combination is mechanism-complementary: probiotic on the gut-immune axis, echinacea on the systemic innate-immune axis. Trial evidence on the combination is limited; each component has its own trial body. Note that S. boulardii is a live-yeast probiotic and is contraindicated in immunosuppressed individuals (transplant patients, biologic DMARDs) for the separate reason of fungaemia risk — the two ingredients share the immunosuppressant contraindication context.
Evidence: Mechanism complementary; combination trials essentially absent.
Doses studied: Echinacea purpurea extract 200-500 mg + S. boulardii 250-1000 mg/day, 7-10 day acute course or as part of broader gut-immune supplementation.
Lactobacillus plantarum / Lactiplantibacillus plantarum (299v / CECT 7484/7485)
Limited evidenceAAD-prevention cluster — different organism domains (yeast vs LAB).
S. boulardii antibiotic-resistant yeast + L. plantarum LAB. Mechanism complementary. Combination products in AAD-prevention context.
Evidence: Each component has individual AAD trial body.
Doses studied: S. boulardii 250-1000 mg + L. plantarum 10⁹-10¹⁰ CFU during antibiotic course.
Lactobacillus reuteri
Moderate evidenceAAD-prevention cluster — different organism domains (yeast vs lactic-acid bacterium).
S. boulardii antibiotic-resistant yeast + L. reuteri lactobacillus. Mechanism complementary; both have AAD prevention + paediatric evidence.
Evidence: Camden saccharomyces-boulardii covers cluster. [11,8,9]
Doses studied: S. boulardii 250-1000 mg + L. reuteri 10⁸-10⁹ CFU daily.