Saccharomyces boulardii

About Saccharomyces boulardii

A yeast rather than a bacterium, isolated from lychee and mangosteen fruit in Indochina in the 1920s. Because it is a fungus, it is not killed by antibacterial drugs.

S. boulardii is one of the few probiotics with review-level evidence across more than one indication: meta-analyses support it for reducing antibiotic-associated diarrhoea, and it has been studied as an adjunct in Clostridioides difficile recurrence and in travellers’ and paediatric diarrhoea. Its indifference to antibiotics is a genuine practical advantage over bacterial probiotics taken during a course of treatment. It is also the probiotic with the best-documented serious harm: fungaemia has been reported repeatedly in critically ill and central-line patients, including through airborne contamination of lines when capsules were opened nearby, and this has caused deaths.

Sold as

Capsule, Sachet of lyophilised yeast, Tablet

Standardised to

Commonly expressed in mg of lyophilised yeast (250–500 mg) or in billions of CFU — a product without this marker may not resemble what was studied.

Also known as

S. boulardii, Brewer’s yeast probiotic, CNCM I-745

Regulation: Registered as a medicine in several European and other markets for acute diarrhoea, and sold as a dietary supplement in the US with no pre-market efficacy review. Product monographs in markets where it is a drug carry explicit warnings against use in critically ill and central-venous-catheter patients.

What the research used

The amounts, forms and durations published trials of Saccharomyces boulardii actually administered.

When it was taken

Each window says whether a trial compared it against an alternative, or whether it is convention and pharmacology. Most timing advice is the second kind.