Saccharomyces cerevisiae, particularly in the form of Saccharomyces cerevisiae var. boulardii (commonly referred to as S. boulardii), has been investigated for its potential to reduce certain side effects of chemotherapy, especially chemotherapy-induced diarrhea. Several randomized controlled trials and meta-analyses suggest that S. boulardii supplementation may decrease the incidence, duration, and severity of diarrhea in patients undergoing chemotherapy, particularly in those treated for colorectal cancer. The proposed mechanisms include restoration of intestinal microbiota balance, modulation of the immune response, and enhancement of gut barrier function.

However, the overall evidence base remains limited, with most studies being small in scale, heterogeneous in design, or focused on adjunctive use rather than standalone therapy. The efficacy appears most notable in cases where chemotherapy regimens are known to disrupt the gut microbiome. S. cerevisiae is not a mainstream standard of care for all chemotherapy patients, but selected guidelines mention consideration of probiotics (including S. boulardii) for prevention or management of gastrointestinal side effects, provided there are no contraindications (such as severe immunosuppression).

There is insufficient evidence to support its use for other chemotherapy side effects (e.g., nausea, mucositis), and safety concerns remain, especially regarding the rare risk of fungemia in immunocompromised patients. Thus, while scientific investigation supports a potential role in reducing gastrointestinal side effects, the overall evidence is moderate and not universally accepted.