Irritable bowel syndrome (IBS) is a common disorder of the intestine, causing abdominal pain with constipation, diarrhea, or alternating periods of both. IBS is also known as spastic colon or spastic bowel (terms that have fallen out of favour now) and functional bowel disorder. It's sometimes mistaken for colitis, an inflammatory bowel disease that is actually a more serious condition involving damage to the colon. IBS doesn't do any damage to the colon.
IBS affects about 20% of the population and is second only to the common cold as the most frequent cause of days lost to work and school. IBS symptoms can disappear for periods of time and then return.
Unlike inflammatory bowel disease (ulcerative colitis and Crohn's disease), IBS brings no extra risk of cancer of the colon. However, if symptoms of IBS begin in people over age 50, it's best to rule out polyps and colon cancer by having a direct inspection of the lower bowel done by a doctor.