Polyps are small growths from the inner lining of the colon. They may cause minor bleeding. This makes screening for blood in the stool a useful tool for early detection of colon cancer. The test can detect small amounts of blood that would typically go unnoticed. Bleeding that occurs higher in the intestine than the colon may be maroon or black.
Peptic ulcers cause black or tarry stools.
Other causes include problems with blood vessels and overuse of alcohol or aspirin. Iron pills cause black stools, but do not cause bleeding. If you notice rectal bleeding, consult your healthcare provider. If your bleeding is nonstop or in large amounts, you should seek emergency care. Tests to find the source of the blood may include x-ray or endoscopy.