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Get information on what a colonoscopy is, how you have it and what happens after it.

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

A doctor or nurse (colonoscopist) uses a flexible tube called a colonoscope. The tube has a small light and camera at one end. The colonoscopist 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 this test

There are a number of reasons why you may need a colonoscopy. These include:

  • to help find the cause of bowel symptoms
  • to look for early signs of bowel cancer as part of the national bowel cancer screening programme
  • to look for early signs of bowel cancer if you are at high risk of developing bowel cancer
  • as part of your follow up after treatment for bowel cancer

Preparing for a colonoscopy

You need to have an empty bowel for the test. So the colonoscopist can clearly see the inside of your bowel. 

You need to take medications (laxatives) to empty your bowel the day before your test. 

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 (not with red or purple colouring)
  • clear soup

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

If you take iron tablets or other medicines that can make you constipated, you might need to stop these for 1 week before the test.

Contact the endoscopy unit as soon as possible before your appointment if you take medicines to thin your blood or you are diabetic.

What happens

You usually have a colonoscopy as an outpatient in the endoscopy department at the hospital. You wear a hospital gown and might also wear paper shorts with an opening at the back. 

You are offered medicine to make you drowsy (sedation) and painkillers. 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 breathed in through a mouth piece. 

Lying on your left side, you draw your knees 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 colonoscopist puts a small bit of gas into your bowel. This makes it easier to see all of the bowel lining. 

Photographs of the bowel lining are taken. The doctor or nurse can remove tissue samples (biopsies). If they see any growths (polyps) they remove them with a small 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 shouldn't drive or drink alcohol for 24 hours after having a sedative. You need someone to take you home from hospital.

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. 

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 gives you 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 freephone 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.

There is a risk of bleeding if you have polyps removed or biopsies taken. This is usually a small amount and stops on its own. Rarely, you might need a blood transfusion or surgery to stop the bleeding.

Very rarely, there can be a small tear (perforation) of the bowel. You would need surgery to repair the tear. 

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

More information

We have more information on tests, treatment and support if you have been diagnosed with cancer.

Last reviewed: 
19 Feb 2016
  • Complications of gastrointestinal endoscopy

    BSG Guidelines in Gastroenterology, 2006

  • NHS Bowel Cancer Screening Programme -  Bowel scope screening

    NHS Bowel Cancer Screening Programme, 2015

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