Colonoscopy is an internal examination of the colon (part of the large intestine) and rectum, using an instrument called a colonoscope. The colonoscope has a small camera attached to a flexible tube. Unlike sigmoidoscopy, which can only reach the lower third of the colon, colonoscopy examines the entire length of the colon.


The patient lies on his or her left side with knees drawn up toward the chest. A sedative and pain reliever is administered, and the colonoscope is inserted through the anus and gently advanced to the lowest part of the small bowel. Air is inserted through the scope to provide a better view. Suction may be used to remove fluid or stool.


Because the colonoscope provides a better view as it is withdrawn, a more careful examination is done while the scope is being pulled out. Tissue samples may be taken with tiny biopsy forceps inserted through the scope. Polyps may be removed with electrocautery snares, and photographs may be taken. Specialized procedures, such as laser therapy, may also be done.


Preparation for colonoscopy

It is essential that the bowel be completely cleansed before colonoscopy. The patient is given instructions for doing this. Preparation may include a combination of enemas, not eating solid foods for 2 or 3 days before the test, and taking laxatives. Patients will usually be told to avoid taking aspirin, ibuprofen, naproxen, or other blood-thinning medications for several days before the test. They will be asked to drink plenty of clear liquids for 1 to 3 days before the test. Examples of clear liquids are: fat-free bouillon or broth, water, clean coffee or tea, strained fruit juices, sports drinks, and gelatin.


Unless otherwise instructed, patients should continue taking any regularly prescribed medication but stop taking iron preparations a few weeks before the test, unless otherwise instructed. Iron residues produce a dark black stool, which makes the view inside the bowel less clear.

People with some heart valve diseases may receive antibiotics before and after the test to prevent infection. Outpatients must plan to have someone take them home after the test, because they will be woozy and unable to drive.


How the test feels

The sedative and pain medication are administered to cause relaxation and drowsiness. Many patients do not remember having the colonoscopy. A rectal examination usually is done before the test to widen (dilate) the rectum and make sure there are no major obstructions. An urge to defecate is normal when the rectal exam is performed or as the colonoscope is inserted.


Pressure may be felt as the scope moves inside, and brief cramping and gas pains as air is inserted or the scope advances. Passing gas is necessary and should be expected. Discomfort can be reduced by taking slow, deep breaths. This will also help relax the abdominal muscles. Mild abdominal cramping and considerable passing of gas may occur after the exam. Sedation should wear off in a few hours. Because of the sedation, patients may not feel any discomfort and may have no memory of the test.


Why the test is performed

Colonoscopy may be used for the following reasons:


  • Anemia (usually when no other cause has been found)
  • Blood in the stool, or black, tarry stools
  • Symptoms such as abdominal pain, changes in bowel movements, or weight loss
  • Abnormal changes (such as polyps) found on sigmoidoscopy or X-ray tests (CT scan or barium enema)
  • Inflammatory bowel disease (ulcerative colitis and Crohn's disease)
  • Follow-up of a previous finding, such as polyps or colon cancer
  • Screen for colorectal cancer

    What abnormal results may indicate


  • Diverticulosis (abnormal pouches on the lining of the intestines, which increase with age)
  • Inflammatory bowel disease
  • Lower gastrointestinal (GI) bleeding
  • Polyps (which can be removed through the colonoscope during the exam)
  • Tumor

    Additional conditions under which the test may be performed:


  • Cytomegalovirus, gastroenteritis, or colitis
  • Colon cancer screening
  • Colorectal polyps
  • Ischemic colitis
  • Pseudomembranous colitis


  • Bowel perforation (a hole or tear in the wall of the colon), requiring a repair operation (between 1 and 3 times out of 1,000 tests)
  • Heavy or persistent bleeding from biopsy or polyp-removal sites (about 7 times out of 1,000 tests in which polyp-removal is done – the larger the polyp, the higher the risk)
  • Adverse reaction to sedative medication, causing breathing problems or low blood pressure (6 to 7 out of 10,000 tests)
  • Infection requiring antibiotic therapy (very rare)
  • Nausea, vomiting, bloating, or rectal irritation caused by medicines, taken by mouth, that cleanse the bowel