Bacillus clausii is a spore-forming probiotic bacterium that has been used, particularly in Europe and Asia, to support gastrointestinal health, including the small intestine. The scientific rationale for its use primarily relates to its ability to survive gastric acidity and reach the small intestine alive, where it can exert beneficial effects. Several clinical trials and systematic reviews have investigated the efficacy of B. clausii in managing and preventing gastrointestinal disorders, most notably antibiotic-associated diarrhea and some forms of acute diarrhea in children and adults.

The evidence suggests that B. clausii may help restore the balance of intestinal microbiota disrupted by antibiotics or infections. It is thought to act through competitive exclusion of pathogens, modulation of immune responses, and production of antimicrobial substances. Some studies have shown reduced duration and severity of diarrhea with B. clausii supplementation, and regulatory agencies in some countries recognize it as a medicinal probiotic.

However, while the evidence base is growing, it is not as robust or comprehensive as for some other probiotics, and results can vary depending on the population studied and the endpoints measured. Most of the evidence pertains to short-term use for acute conditions rather than long-term support of small intestine health in otherwise healthy individuals. Thus, while the use of B. clausii to support the small intestine is scientifically justified, the strength of evidence is moderate.