Saccharomyces cerevisiae, particularly the strain Saccharomyces cerevisiae var. bouardii (often referred to as Saccharomyces boulardii), has been scientifically studied and is widely used as a probiotic for the prevention and treatment of certain types of diarrhea. Multiple randomized controlled trials and meta-analyses have demonstrated its efficacy, especially in the context of antibiotic-associated diarrhea (AAD) and acute infectious diarrhea in both adults and children. The proposed mechanisms of action include inhibition of pathogenic bacteria, modulation of host immune responses, and reinforcement of the intestinal barrier function.

S. cerevisiae var. bouardii is unique among probiotics because it is a non-pathogenic yeast, which means it is unaffected by antibiotics that target bacteria, making it particularly useful during antibiotic therapy. Clinical guidelines such as those from the World Gastroenterology Organisation and the European Society for Paediatric Gastroenterology, Hepatology and Nutrition (ESPGHAN) endorse its use for reducing the duration and severity of acute diarrhea and for preventing AAD.

It is important to note that while S. cerevisiae (baker's yeast) is not commonly used for this purpose, the closely related probiotic yeast S. cerevisiae var. bouardii is the strain with robust evidence. Its use is supported by high-quality clinical studies, though it should be used with caution in immunocompromised individuals due to rare cases of fungemia.