Colonoscopy is a method for inspecting the surface of the colon (also called the large bowel). Although colonoscopy has many uses, the most common use is to screen for colon cancer, to investigate symptoms of colon cancer (such as blood in the stool), to prevent colon cancer (by removing polyps), and to check for recurrence after removing polyps or after surgery for colon cancer. Colonoscopy is a safe test, but things occasionally go wrong. The most common problem is bleeding after biopsy of a polyp or cancer. A more rare but more serious problem is when a hole is punched in the wall of the colon (a perforation); the complication occurs about 3 times in every 1000 colonoscopies.