A colonoscopy is the examination of the inside of the colon using a colonoscope, a flexible camera instrument passed through the rectum. It is used to investigate symptoms such as bleeding, persistent changes in bowel habit, and abdominal pain, and as a screening tool for colorectal cancer, during which precancerous polyps can be removed on the spot. Preparation involves a clear-fluid diet and strong laxatives to empty the bowel completely. Though invasive and uncomfortable, it is the most thorough colonic investigation available, and its role in preventing bowel cancer through polyp removal saves many lives.