The pharmacology, uses, dosages, safety, drug interactions, and contraindications of probiotics are reviewed.Probiotics are live nonpathogenic microorganisms administered to improve microbial balance, particularly in the gastrointestinal tract. They consist of Saccharomyces boulardii yeast or lactic acid bacteria, such as Lactobacillus and Bifidobacterium species, and are regulated as dietary supplements and foods. Probiotics exert their beneficial effects through various mechanisms, including lowering intestinal pH, decreasing colonization and invasion by pathogenic organisms, and modifying the host immune response. Probiotic benefits associated with one species or strain do not necessarily hold true for others. The strongest evidence for the clinical effectiveness of probiotics has been in the treatment of acute diarrhea, most commonly due to rotavirus, and pouchitis. More research is needed to clarify the role of probiotics for preventing antibiotic-associated diarrhea, Clostridium difficile infection, travelers' diarrhea, irritable bowel syndrome, ulcerative colitis, Crohn's disease, and vulvovaginal candidiasis. There is no consensus about the minimum number of microorganisms that must be ingested to obtain a beneficial effect; however, a probiotic should typically contain several billion microorganisms to increase the chance that adequate gut colonization will occur. Probiotics are generally considered safe and well tolerated, with bloating and flatulence occurring most frequently. They should be used cautiously in patients who are critically ill or severely immunocompromised or those with central venous catheters since systemic infections may rarely occur. Bacteria-derived probiotics should be separated from antibiotics by at least two hours.Probiotics have demonstrated efficacy in preventing and treating various medical conditions, particularly those involving the gastrointestinal tract. Data supporting their role in other conditions are often conflicting.