Constipation: Infrequent and frequently incomplete bowel movements. Constipation is the opposite of diarrhea and is commonly caused by irritable bowel syndrome (IBS), diverticulosis, and medications. Paradoxically, constipation can also be caused by overuse of laxatives. Colon cancer can also narrow the colon and thereby cause constipation. A high-fiber diet can frequently relieve constipation. If the diet is not helpful, medical evaluation is warranted.