Colorectal cancer is cancer of the colon (large intestine) or rectum. Screening decreases deaths from colorectal cancer by detecting precancerous abnormalities (polyps) and removing them before they become cancer and by detecting cancer in an early stage when it can still be cured with surgery. Screening tests include fecal occult blood testing (FOBT), flexible sigmoidoscopy, and colonoscopy. FOBT uses a chemical reaction to detect blood hidden in stool. Polyps and colorectal cancers can cause blood to leak into the stool, so a positive result on FOBT suggests the need for further testing. Flexible sigmoidoscopy involves looking into the rectum and lower colon through a tube-shaped instrument; colonoscopy uses a similar but longer instrument to look at the entire length of the colon. While it is not clear which screening strategy makes the most sense in terms of costs and associated benefits, it is clear that some form of routine screening is favorable after age 50 in terms of health benefits and economics. Taking an aspirin a day may also help to prevent colorectal cancer. However, the degree of benefit from aspirin is unclear, and aspirin can cause bleeding complications.