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 gastrointestinal disorder (FGID), 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),or a stressful life event, but varies little with age.