Saccharomyces boulardii: Antibiotic Diarrhea, C. difficile Evidence and Safety Cautions

Read this first: who should be careful

Saccharomyces boulardii is a live yeast. Rare bloodstream infections with this yeast (fungemia) have been reported. A Belgian review of 10 patients with S. cerevisiae fungemia found that 70% had a central venous line, 30% were in intensive care and half died; 6 had been given a probiotic containing S. boulardii. [1] The authors called it rare but life-threatening and suggested older age should count as a reason to avoid these probiotics. [1] A separate report described yeast bloodstream infection in three intensive care roommates of patients receiving a freeze-dried preparation, probably through catheter contamination, and stressed strict hygiene. [2]

NCCIH says the risk of harm is greater in people with severe illness or weakened immunity. [3] If you or someone you care for has a central line, is critically ill, is immunocompromised or has had a recent hospital stay, ask the treating team before using this yeast. See probiotics and bloodstream infection.

Diarrhea that is bloody, lasts more than 7 days, or comes with signs of dehydration needs medical advice first; so does diarrhea in an older person or someone with weakened immunity. [4] Do not use a probiotic as a reason to postpone seeing a clinician. [3]

What it is

NCCIH lists Saccharomyces boulardii among yeasts used in probiotics, alongside the bacteria. [3] WGO notes that most studies used the strain S. boulardii CNCM I-745. [5] Some papers call the organism S. cerevisiae subtype boulardii. [2] Always match a product's strain code to the study you read.

Evidence labels used below: promising but limited, mixed, insufficient evidence. They are our plain summaries, not the sources' grades. "CoE" is the certainty of evidence as graded by the AGA reviewers.

What the research found, by use

The 2018 McFarland review of 228 trials found the CNCM I-745 yeast worked differently across diseases, so one result should not be carried to another condition. [11] Treatment of C. difficile, IBD and H. pylori is led by clinicians; the studies above added a probiotic to treatment, not in place of it. [6]

Amounts used in studies

The CDI trial gave 1 g (3 x 10^10 CFU) a day with antibiotics. [6] WGO's tables list other trial amounts for adults and children. [5] These are study amounts, not advice, and the AGA did not set a best amount.

Side effects and other cautions

• In the AGA review the usual adverse events were mild digestive symptoms; one trial found more constipation with S. boulardii than placebo, and no serious events were reported. [6]
• Across all probiotics in the 2025 Cochrane review the common events were cramping, nausea, fever, soft stools, gas and taste change; the review applied to people who were not immunocompromised. [7]
• Quality: the NHS says probiotic labels cannot always be trusted to match contents. [12] See quality testing and certifications.
• Medicines: antibiotics do not typically kill yeast, per the AGA. [6] Interactions with antifungal or immune-suppressing medicines are not described in the sources we read, so ask a pharmacist; see medication interactions.
• Proper evidence not found for S. boulardii in pregnancy, breastfeeding, kidney or liver disease or before surgery (searched: NCCIH, NHS, AGA review, WGO guideline). See pregnancy and breastfeeding and supplements before surgery.
• Premature infants: NCCIH reports FDA's 2023 warning about severe infections; decisions belong to the neonatal team. [3]

More: antibiotic-associated diarrhea, C. difficile.