Probiotics are live microorganisms, usually bacteria or yeasts, that when consumed in adequate amounts can confer health benefits. Most research focuses on strains of Lactobacillus, Bifidobacterium and Saccharomyces. Effects are strongly strain specific, so findings for one strain cannot be generalized to all probiotics.

Evidence is clearest for certain digestive conditions. Specific strains can shorten the duration of acute infectious diarrhea, especially in children, and may modestly reduce the risk of antibiotic associated diarrhea, including some cases caused by C. difficile. Some probiotic preparations help maintain remission in ulcerative colitis or pouchitis, while benefits for Crohn’s disease remain uncertain. For irritable bowel syndrome, several strains show small to moderate improvements in pain, bloating and stool consistency, but responses vary widely between individuals.

Probiotics can influence the immune system by interacting with gut associated lymphoid tissue and by producing metabolites that affect inflammation. Some studies report reduced risk or milder course of respiratory and common gastrointestinal infections, though effects are generally modest. There is preliminary evidence that certain strains may help manage atopic dermatitis in children or reduce the risk of necrotizing enterocolitis in preterm infants.

Safety is good for healthy people, with mostly mild side effects like gas and bloating. However, in severely ill or immunocompromised patients there are rare reports of bloodstream infections, so medical supervision is important.

Research is moving toward personalized approaches, combining probiotic strains with diet, prebiotics and microbiome profiling to better target who will benefit and how.