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A colonoscopy is a procedure that lets the doctor look at the lining of the entire rectum and colon using an endoscope (colonoscope) with a camera on the end. (Sigmoidoscopy examines only the rectum and lower third of the colon.)

Why colonoscopy is done

A colonoscopy may be done to:

  • look for causes of:
    • blood in the stool
    • anemia
    • abdominal pain
    • changes in bowel habits, such as constipation or diarrhea
  • look for areas of inflammation that may indicate ulcerative colitis or Crohn’s disease
  • follow-up after other tests or treatments:
    • look for abnormal areas in the lining of the colon that were identified with double-contrast barium enema
    • look for additional polyps if any were found by sigmoidoscopy or previous colonoscopy
    • ensure complete removal of a large polyp
    • recheck the colon following surgery for colon cancer
  • obtain biopsy tissue for examination
  • screen for colon cancer in high-risk individuals, including people with:
    • a family history of colon cancer
    • a history of benign polyps
    • a history of familial adenomatous polyposis (FAP) or Lynch syndrome (also called hereditary non-polyposis colorectal cancer, or HNPCC)
  • diagnose invasive cancer

How colonoscopy is done

A colonoscopy is done in a hospital on an outpatient basis. The procedure usually takes about 30–45 minutes, but it can take longer.

  • Special preparation is needed before the doctor can perform a colonoscopy.
    • The person cannot eat solid foods for 1–2 days before the test.
    • A person will be given instructions on what food or liquid they can have before the test.
    • They will also be given instructions on how to flush out the colon before the test (using a laxative, enema or both) to ensure it is empty. This will make the person go to the bathroom often.
  • The person will need to take off their clothes and wear a gown during the test.
  • The person lies on one side.
  • An intravenous line will be placed in the arm so that medications, including sedation, pain medication and antibiotics, can be given during the procedure.
  • The doctor inserts the endoscope through the anus and into the rectum and colon.
  • The colon is inflated with air to stretch out the lining so the doctor can inspect the entire surface.
  • Instruments can be passed through the endoscope to perform biopsies, remove polyps and stop bleeding.
    • Removal of polyps is called polypectomy.
  • During and after the test, it is normal to have some mild abdominal cramping and to pass gas.

The person is monitored for 1–2 hours after the test.

  • The person will not be able to drive for 12 hours after the test.
  • Check with the doctor about when you can resume your normal diet and activities.

Potential side effects

Common side effects of colonoscopy include:

  • small amounts of blood in the stool for 1–2 days, if a biopsy was done or if a polyp was removed
  • nausea, vomiting, bloating or irritation of the rectum due to the substances used to cleanse the colon and the air used to inflate it

Uncommon but serious complications include:

  • bowel perforation
    • The person will be admitted to hospital and given antibiotics.
    • Sometimes surgery is needed to repair the perforation.
  • heavy bleeding
    • If the bleed is severe, the person may require:
      • blood transfusion
      • repeat colonoscopy
      • surgery to stop the bleeding
  • allergic reaction to the medication used for sedation

What the results mean

An abnormal result may indicate:

  • diverticulosis (abnormal pouches in the lining of the colon)
    • Diverticulosis is most often seen in older people.
    • It is most common in the sigmoid colon.
  • inflammatory bowel disease (ulcerative colitis or Crohn’s disease)
  • bleeding in the colon or rectum
  • polyps
  • cancer of the colon or rectum

What happens if a change or abnormality is found

If a polyp cannot be removed during the colonoscopy, the person will require further treatment. This treatment usually involves removing a section of the colon along with a margin of normal-looking tissue (bowel resection). Depending on the type of surgical procedure used, a colostomy may be needed.

Biopsy samples and polyps are sent to the pathology lab. A pathologist will look at the cells in the tissue to see if they contain cancer. If cancer is found, the person will undergo additional tests to help determine how advanced the cancer is.


A thin, tube-like instrument with a light and lens used to examine or treat organs or structures in the body.

An endoscope can be flexible or rigid. It may have a tool to remove tissue for examination. Specialized endoscopes may have tools designed to examine or treat specific organs or structures in the body.

Specialized endoscopes are named for the organ or structure they are used to examine or treat.


A reduction in the number of healthy red blood cells.


A procedure that uses an endoscope (a thin, tube-like instrument with a light and lens) to examine or treat sigmoid colon (the last part of the colon) and rectum.

Cells or tissue may be removed for examination under a microscope. Doctors may also use sigmoidoscopy to control bleeding or remove polyps or tumours.

The type of endoscope used for this procedure is called a sigmoidoscope. Doctors generally use flexible sigmoidoscopes because they give more complete views of the sigmoid colon and are more comfortable than rigid sigmoidoscopes.

Also called proctosigmoidoscopy.


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