What is a colonoscopy? A colonoscopy allows a doctor to look inside the entire large intestine.The procedure enables the physician to see things such as inflamed tissue, abnormal growths, and ulcers. It is most often used to look for early signs of cancer in the colon and rectum. It is also used to look for causes of unexplained changes in bowel habits and to evaluate symptoms like abdominal pain, rectal bleeding,and weight loss. The colon, or large bowel, is the last portion of your digestive tract, or gastrointestinal tract.