Colonoscopy for bowel cancer

A colonoscopy looks at the whole of the inside of your large bowel.

An endoscopist (a specially trained healthcare professional) uses a flexible tube called a colonoscope. The tube has a small light and camera at one end. The endoscopist puts the tube into your back passage and passes it along the bowel. They can see pictures of the inside of your bowel on a TV monitor. 

Why you might have a colonoscopy

You might have a colonoscopy to:

  • help find the cause of your bowel symptoms
  • look for early signs of bowel cancer as part of the national screening programme, or if you are at high risk of bowel cancer
  • monitor you after treatment for bowel cancer
Diagram showing a colonoscopy

Before your test

Your bowels need to be empty for a colonoscopy.

To do this you take medications (laxatives) to empty your bowel the day before your test. A laxative is a liquid bowel preparation.

After taking the laxatives you might need the toilet often and very suddenly. You might have some cramps. It is sensible to stay at home for a few hours after taking laxatives so that you are near a toilet. 

The hospital might ask you to eat a low fibre diet for 1 or 2 days before the test. It is very important to drink plenty of clear fluids. This includes:

  • water
  • black tea or coffee
  • squash (without red or purple colouring)
  • clear soup

You should get written instructions before your test about what you need to do.

Contact the hospital for advice if you are diabetic or taking regular medication. You might need to stop some medicines before your test.

What happens

You usually have a colonoscopy as an outpatient in the endoscopy department at the hospital.

Before the procedure you’ll see your endoscopist. They'll explain the procedure to you and ask you to sign a consent form. This is a good time to ask any questions you may have. 

You wear a hospital gown and might also wear paper shorts with an opening at the back. 

You're offered painkillers and a medicine to relax you, these make you drowsy (sedation). You have these as injections into a small tube (cannula) in your vein. Some hospitals use gas and air (Entonox). This is a fast acting pain relief that you breathe in through a mouth piece. 

You lie on your left side with your knees drawn up towards your chest. The colonoscope goes into your back passage (rectum) and passes into your bowel. The tube bends easily so it can pass around the curves in your bowel.

The endoscopist puts a small amount of gas is put into your bowel to help see all the bowel lining. 

The endoscopist may press on your stomach or change your position to help the tube pass through your bowel. 

During the test they take photographs of your bowel lining. And the endoscopist can remove tissue samples (biopsies). If they see any growths (polyps) they remove them with a wire loop put down the colonoscope. You might also have a dye sprayed onto the lining of the bowel to show up any abnormal areas. This is called chromoscopy. 

Colonoscopies can be uncomfortable but shouldn't be painful.

After your test

You go to a recovery area to rest if you have had sedation. Your nurse monitors you. They offer you a snack and drink when you are ready. You might be in the recovery area for a couple of hours.

You might have bloating and cramping pains after the test. This is due to the gas that goes into your bowel. It normally gets back to normal after a few hours. 

As you’re having sedation you’ll need someone with you so they can take you home and stay with you overnight. Also for 24 hours after you shouldn’t drive, drink alcohol, operate heavy machinery or sign any legally binding documents.

Getting your results

Before you go home, your doctor tells you if they removed any growths (polyps) or tissue samples (biopsies) from your bowel. 

The biopsy results can take up to 2 weeks. Your specialist writes to you with the results. If your GP referred you for the test, they should also receive a copy. Contact your doctor if you haven’t heard anything after a couple of weeks.

Waiting for biopsy results can be an anxious time. It might be helpful to talk to someone close to you.

You can also contact the Cancer Research UK nurses on a freephone number 0808 800 4040. The lines are open from 9am to 5pm, from Monday to Friday.

Possible risks

Colonoscopy is a very safe procedure. But your nurse will tell you who to contact if you have any problems afterwards. Your doctors make sure the benefits of having a colonoscopy outweigh any possible risks.

Some of the possible risks include:

Tummy (abdominal) pain

You may have some cramping or pain in your tummy after the test. This is due to the carbon dioxide or air they put into the bowel. This should go away after a few hours. You may also experience some pain or tenderness if you had some tissue removed during the colonoscopy. 


After having a biopsy or polyp removed, you might have some bleeding. This is usually a small amount and stops on its own within a couple of days.

Tear in your bowel

Very rarely there is a small tear in the bowel wall (perforation). If this happens it’s likely you would need surgery to repair the tear.

Difficulty breathing

There is a risk you might react to the sedation drug. It can cause temporary feeling of difficulty with breathing. But this is rare. It's easily treated by giving you oxygen through a small plastic tube into your nose. Your nurse monitors you closely.  

  • Association of Coloproctology of Great Britain & Ireland (ACPGBI): Guidelines for the management of cancer of the colon, rectum and anus (2017) – diagnosis, investigations and screening
    C Cunnigham and others
    Colorectal disease, 2017. Volume 19, Pages 1-97

  • Localised colon cancer: ESMO Clinical Practice Guidelines for diagnosis, treatment and follow-up.
    G Argilés and others
    Annals of Oncology, 2020. Volume 31, Issue 10 Pages 1291-1305

Last reviewed: 
06 Dec 2021
Next review due: 
06 Dec 2024

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