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Probiotics & Digestive

Saccharomyces Boulardii

Also known as: S. boulardii, Boulardii probiotic, Florastor

The antibiotic-proof probiotic — a yeast, not a bacterium, with genuine diarrhea-prevention evidence.

  • By IngredientWiki Editorial Team
  • Fact-checked by IngredientWiki Editorial Team
  • Updated 2026-09-26
  • 2 min read

In short

Saccharomyces boulardii is a probiotic with a taxonomic twist: it is a yeast, not a bacterium — which gives it two practical superpowers. It survives stomach acid without enteric coating and, crucially, antibiotics cannot kill it, so it can be taken during antibiotic courses precisely when bacterial probiotics would be destroyed. The clinical record is correspondingly strong: S. boulardii is among the best-evidenced probiotics for preventing antibiotic-associated diarrhea, with meta-analyses showing roughly halved risk, and it has solid data in traveler's diarrhea and recurrent C. difficile. It is the probiotic to reach for when antibiotics are involved.

  • A probiotic yeast (not bacteria) — survives stomach acid
  • Unaffected by antibiotics — taken during antibiotic courses
  • Meta-analyses: ~halved antibiotic-associated diarrhea risk
  • Also used for traveler's diarrhea and C. difficile recurrence

What is Saccharomyces Boulardii?

S. boulardii breaks the probiotic mold in a way that matters clinically: as a yeast, it laughs off the antibiotics that annihilate bacterial probiotics — which is exactly when it is needed most. The evidence has rewarded the biology: pooled trials consistently show S. boulardii roughly halves the risk of antibiotic-associated diarrhea, and it is one of the few probiotics with meaningful C. difficile data. It is the specialist of the probiotic category — not a daily gut-balance tonic so much as the targeted tool for the specific situations where diarrhea risk is highest.

Where it comes from

Natural sources

  • A yeast strain (Saccharomyces cerevisiae var. boulardii), first isolated from lychee and mangosteen skins in 1920s Indochina
  • Discovered when a French scientist observed locals drinking a tea that prevented cholera diarrhea
  • Sold as a lyophilized (freeze-dried) preparation that reactivates in the gut

How it's made

Commercial products deliver the live yeast in capsules or sachets (e.g., Florastor), typically 250 mg per dose.

What it does in products

  • Survives gastric acid — reaches the gut alive
  • Secretes factors that neutralize bacterial toxins
  • Competes with pathogens for gut-surface binding
  • Resistant to all antibiotics — combinable with treatment

Benefits

Antibiotic-associated diarrhea

strong evidence

Meta-analyses of randomized trials find S. boulardii roughly halves the risk of diarrhea during and after antibiotic courses — among the strongest results in all of probiotics.

C. difficile recurrence

moderate evidence

Trials support S. boulardii as an adjunct in reducing recurrent Clostridioides difficile infection, a niche few probiotics can claim.

Traveler's diarrhea

moderate evidence

Studies show reduced incidence and severity of traveler's diarrhea with preventive dosing — useful, though not a substitute for food-water caution.

General gut support

emerging evidence

Less evidence as a daily general probiotic — its strengths are situational rather than maintenance-oriented.

Side effects & safety

Gas or bloating (uncommon)Usually mild and transient.
Systemic fungal infection (rare)Serious risk limited to immunocompromised patients and those with central venous catheters — avoid in those settings.

Safety considerations

  • Generally considered acceptable in pregnancy (it is not absorbed), but confirm with your provider.
  • Can be taken with antibiotics (its defining advantage); no significant drug interactions.
  • Not for: Severe immunocompromise
  • Not for: Central venous catheters
  • Not for: Known yeast allergy

How it's used

In supplements

Typical: 250–500 mg (5–10 billion CFU) once or twice daily. For antibiotics, start with the course and continue several days after it ends. For traveler's diarrhea, start 2–5 days before travel.

Regulatory status

  • Dietary supplement under DSHEA; no drug approval in the US. Widely used as an adjunct therapy in European medicine.
  • GRAS (generally recognized as safe) for food use.

Popular products containing Saccharomyces Boulardii

  • Florastor (S. boulardii) — The reference productBiocodex · Supplement
  • Saccharomyces BoulardiiJarrow Formulas · Supplement
  • S. Boulardii CapsulesNOW Foods · Supplement

Product names and brands are shown for reference and belong to their respective owners.

Why a yeast outperforms bacteria during antibiotics

The antibiotic-diarrhea niche exposes the structural flaw of bacterial probiotics: take them during an antibiotic course and the drug kills the very organisms you swallowed. S. boulardii sidesteps the problem entirely — as a yeast, it is immune to antibacterial antibiotics, survives stomach acid, and colonizes transiently while secreting factors that neutralize the toxins pathogens leave behind. The clinical result is the best evidence base in probiotic diarrhea prevention. The design lesson generalizes: in probiotics, matching the organism to the situation beats matching the marketing to the audience.

Frequently asked questions

Can S. boulardii be taken with antibiotics?

Yes — that is its defining advantage. As a yeast it is unaffected by antibacterial drugs, so it can be taken throughout the course and continued for several days afterward to prevent diarrhea.

Does S. boulardii help C. difficile?

As an adjunct to medical treatment, trials support reduced recurrence of C. difficile infection. It is supportive care — C. diff itself requires physician management.

Is S. boulardii safe for everyone?

Mostly, but the exception is serious: immunocompromised patients and those with central venous catheters have rare cases of systemic yeast infection and should avoid it. Otherwise, side effects are mild.

The bottom line

S. boulardii is the probiotic specialist for the antibiotic era: antibiotic-proof, acid-proof, and backed by the strongest diarrhea-prevention data in the category. Reach for it during antibiotic courses and before travel — and keep it away from immunocompromised patients.

References

  1. S. boulardii and antibiotic-associated diarrhea (meta-analysis). PubMed (National Library of Medicine). https://pubmed.ncbi.nlm.nih.gov/?term=saccharomyces+boulardii+antibiotic+diarrhea+meta
  2. S. boulardii and C. difficile — literature search. PubMed (National Library of Medicine). https://pubmed.ncbi.nlm.nih.gov/?term=saccharomyces+boulardii+clostridium+difficile
  3. S. boulardii and traveler's diarrhea — literature search. PubMed (National Library of Medicine). https://pubmed.ncbi.nlm.nih.gov/?term=saccharomyces+boulardii+travelers+diarrhea
  4. S. boulardii mechanism (review). PubMed (National Library of Medicine). https://pubmed.ncbi.nlm.nih.gov/?term=saccharomyces+boulardii+mechanism+review