Probiotics are live microorganisms, mainly specific strains of bacteria and yeasts, that can benefit health when consumed in adequate amounts. Most research focuses on strains like Lactobacillus, Bifidobacterium and the yeast Saccharomyces boulardii. Effects are highly strain specific, so results from one strain do not automatically apply to others.

Evidence is strongest for digestive health. Certain probiotic strains can shorten the duration of infectious diarrhea, help prevent antibiotic associated diarrhea and modestly reduce symptoms of irritable bowel syndrome, such as abdominal pain and bloating. Some preparations appear to support remission maintenance in ulcerative colitis, although findings are mixed and disease specific.

Probiotics can influence the immune system by interacting with gut associated immune cells and by changing microbial communities. Some studies show fewer respiratory infections or milder symptoms in people taking particular strains, and there is emerging evidence for benefits in some allergic conditions, especially in infants at high risk of eczema. However, results are inconsistent and depend heavily on strain, dose and timing.

Research also links gut microbes with brain function, leading to interest in so called psychobiotics. Early trials suggest that some probiotics may modestly affect mood or stress related symptoms, but the evidence is still preliminary and mechanisms are not fully understood.

Safety is generally good for healthy people, with mostly mild side effects like gas or discomfort. However, serious infections, though rare, have occurred in severely ill or immunocompromised individuals. Overall, probiotics show real but targeted benefits, and understanding which specific strains help which conditions remains an active area of research.