Irritable bowel syndrome (IBS, or spastic colon) is a symptom-based diagnosis characterized by chronic abdominal pain, discomfort, bloating, and alteration of bowel habits. As a functional bowel disorder, IBS has no known organic cause. Diarrhea or constipation may predominate, or they may alternate (classified as IBS-D, IBS-C or IBS-A, respectively). Historically a diagnosis of exclusion, a diagnosis of IBS can now be made on the basis of symptoms alone, in the absence of alarm features such as age of onset greater than 50 years, weight loss, gross hematochezia, systemic signs of infection or colitis, or family history of inflammatory bowel disease. Onset of IBS is more likely to occur after an infection (post-infectious, IBS-PI), a stressful life event, or onset of maturity.

Although there is no cure for IBS, there are treatments that attempt to relieve symptoms, including dietary adjustments, medication and psychological interventions. Patient education and a good doctor-patient relationship are also important.

Several conditions may present as IBS including coeliac disease, fructose malabsorption, mild infections, parasitic infections like giardiasis, several inflammatory bowel diseases, bile acid malabsorption, functional chronic constipation, and chronic functional abdominal pain. In IBS, routine clinical tests yield no abnormalities, although the bowels may be more sensitive to certain stimuli, such as balloon insufflation testing. The exact cause of IBS is unknown. The most common theory is that IBS is a disorder of the interaction between the brain and the gastrointestinal tract, although there may also be abnormalities in the gut flora or the immune system.