Ulcerative colitis is an inflammatory bowel disease (IBD) that causes ongoing inflammation and ulcers (open sores) in the innermost layers of the large intestine (colon) and rectum. The ulcers bleed and produce pus and mucus, and the inflammation causes the colon to empty frequently, causing diarrhea.

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Ulcerative colitis is similar to Crohn disease, another IBD. Crohn disease can happen anywhere in the digestive tract, often in patches, and can spread deeper into tissues.

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Ulcerative colitis, on the other hand, is usually confined to the innermost layers of tissue and is uniform throughout the colon. Ulcerative colitis can be painful and have life-threatening complications.