Probiotics are live microorganisms that, when administered in adequate amounts, confer a health benefit on the host. Their use in Inflammatory Bowel Disorders (IBD), which mainly include Crohn's disease and ulcerative colitis, is supported by a moderate amount of scientific evidence, although findings are variable depending on the specific condition and probiotic strain used.

Clinical trials have shown that certain probiotic strains, particularly Escherichia coli Nissle 1917 and multi-strain preparations like VSL#3, can be effective in maintaining remission in ulcerative colitis and in preventing pouchitis (inflammation of the ileal pouch) following surgery. For example, a 2010 Cochrane review concluded that probiotics are more effective than placebo for maintaining remission in ulcerative colitis, though less effective than standard therapy (mesalazine). In contrast, evidence for the efficacy of probiotics in Crohn's disease is weak, with most studies showing no significant benefit over placebo.

The proposed mechanisms involve restoring gut microbial balance, enhancing intestinal barrier function, and modulating immune responses. However, results vary significantly by strain, dosage, and patient population. Current clinical guidelines suggest selective use of probiotics, mainly for ulcerative colitis and pouchitis, but not for Crohn's disease. More large-scale, high-quality trials are needed to define optimal strains, combinations, and treatment protocols.