Toxic megacolon is a rare but dangerous condition that occurs when the colon swells to many times its normal size. It is usually a complication of an inflammatory bowel disease, such as ulcerative colitis or Crohn's disease.

Severe inflammation and ulceration can weaken muscles in the colon, causing the colon to swell. Symptoms may include a swollen belly, abdominal pain or tenderness, rapid heartbeat, or fever. Over time, holes (perforations) may form in the colon, and stool may spill into the abdominal cavity, causing a serious infection. This can be life-threatening.

Toxic megacolon is an emergency that requires immediate medical treatment to prevent dehydration and shock. Surgery may be needed to remove all or part of the colon (colectomy).