A colonoscope is used to examine the lining of the bowel for abnormalities. This test is called a colonoscopy and is the most effective way to diagnose bowel cancer.

The bowel needs to be completely empty, so you will need to follow a careful diet before the test. You will also need to take a laxative. The department carrying out your colonoscopy will give you information about this. 

Before the colonoscopy, you may be given a sedative to help you relax. Once you are lying on your side, a thin, very flexible tube (colonoscope) is passed gently into your back passage. Photographs and samples of cells on the inside of the large bowel may also be taken for examination. Most colonoscopies are problem free, but complications are possible.

It can take up to three weeks before the results of your colonoscopy are ready. This can be an anxious time. Try speaking with a close friend or relative about your thoughts and feelings.

Having a colonoscopy

A colonoscopy is a way of examining the lining of the bowel from the inside. The procedure is usually done in the hospital outpatient department and takes about an hour. It's the most thorough and effective way of diagnosing bowel cancer. The colonoscopy can also see if there are any polyps in your bowel.

The bowel has to be completely empty for a colonoscopy. This means following a careful diet for a few days before your test. You'll also need to take laxatives, which the screening centre or hospital pharmacy will provide. The department carrying out your colonoscopy will give you instructions about your diet, and about exactly when to take your laxatives.

Shortly before the colonoscopy, you may be given a sedative to help you feel relaxed. Once you're lying comfortably on your side, the nurse or doctor will gently pass a thin flexible tube (a colonoscope) into your back passage. The tube is made up of flexible fibres so it can easily pass around the curves of the bowel. A tiny light and camera on the end of the tube are used to show any abnormal areas.

During the colonoscopy, photographs and samples (biopsies) of the cells on the inside of the large bowel can be taken. Most polyps can be painlessly removed using a wire loop that is passed down the colonoscope. A colonoscopy can be uncomfortable, but the sedative will help you feel more relaxed.

Most people are ready to go home a couple of hours after their test. You'll need to arrange for someone to collect you from the hospital as you shouldn't drive for several hours after a sedative. You should also have someone with you for about 12 hours afterwards.

Sometimes it's not possible to see the whole bowel during a colonoscopy. This can happen if the bowel isn't completely empty or if the colonoscope can't pass round a bend in the bowel to reach the end. If this happens, you may be asked to have another colonoscopy, or a CT colonogram.

Having a colonoscopy

Dr Ana Wilson, Consultant Gastroenterologist, explains what to expect if you're having a colonoscopy.

About our cancer information videos

Having a colonoscopy

Dr Ana Wilson, Consultant Gastroenterologist, explains what to expect if you're having a colonoscopy.

About our cancer information videos

Potential complications or risks of a colonoscopy

For most people a colonoscopy is a straightforward procedure and they'll have no side effects. Rarely however, complications can happen, including:

Fluid loss

Taking laxatives before having a colonoscopy may sometimes cause you to lose a lot of fluid from your body, as you pass several bowel motions. If you have heart problems, let your GP or the screening centre know before you take any laxatives as this fluid loss can temporarily worsen your condition.

Breathing or heart problems

Some people react to the sedative that is used during a colonoscopy. This can cause temporary breathing or heart problems. It's rare to have a serious problem and you'll be monitored during the colonoscopy.

Heavy bleeding

About 1 in every 250 people who have a colonoscopy will have heavy bleeding afterwards. If tissue samples (biopsies) are taken or polyps removed there is a risk that the area may bleed. If you notice a lot of bleeding, contact the screening centre as soon as possible for further advice.

A perforated bowel

Rarely the colonoscope can make a hole (perforation) in the wall of your bowel. This happens to about 1 in 1,000 people who have a screening colonoscopy. If your bowel is perforated you may need an operation to mend the hole.

Risk of death

In extremely rare cases, a person may die as a result of having a colonoscopy. This is very rare and only happens in around 1 in every 10,000 colonoscopies. The benefits of having a colonoscopy far outweigh this risk.

Results of your colonoscopy

The specialist doing your colonoscopy will tell you if they have removed any tissue samples (biopsies) or polyps from your bowel. If they have taken samples, you'll have to wait up to three weeks for the results.

This can be an anxious time for you and it may help to talk things over with a relative or close friend, or one of these organisations.

The possible results you could get include:

  • a normal result
  • benign polyps
  • other benign causes, such as inflammatory bowel disease
  • a cancer.

Normal result

This means that no polyps or cancer have been found in your bowel. About half of people who have a colonoscopy will get a normal result.

There is a small chance that the colonoscopy may miss a cancer, so a normal result doesn't guarantee that you don't have cancer, or won't develop it in the future. If your colonoscopy was normal you'll be offered bowel cancer screening again in two years, as long as you're still within the invitation age range. If you are older than this you can continue to be screened every two years by requesting a screening kit.

Benign polyps

If one or more polyps are found during your colonoscopy, they can usually be removed at the same time. This is known as a polypectomy and it can help to prevent bowel cancer developing. Around 4 in 10 (40%) of people who have a colonoscopy following an abnormal FOB test are found to have polyps.

The removed polyps will be looked at in the laboratory and, depending on the results, you may be invited to continue with the two-yearly bowel cancer screening or have another colonoscopy, usually in one or three year's time.

In some cases, polyps may come back again after surgery.

Other benign causes, such as inflammatory bowel disease

If your colonoscopy results show that you have inflammatory bowel disease, such as Crohn's disease or ulcerative colitis, you'll usually be referred to a gastroenterologist, a doctor who specialises in these conditions.

A cancer

Only very few people (about 1 in 10, or 10%) who have a colonoscopy after an abnormal FOB test will be diagnosed with bowel cancer. If a cancer is found, you'll be referred to a cancer specialist for treatment.

If a cancer is found at its earliest stage, there is a very good chance of curing it. About 9 out of every 10 (90%) early-stage bowel cancers can be successfully treated. However, not all bowel cancers that are found by colonoscopy can be cured.

If you are diagnosed with cancer in the colon or rectum you may find it helpful to read our booklets Understanding colon cancer and Understanding rectal cancer.

How reliable are colonoscopy results?

Although having a colonoscopy is the best way to diagnose bowel cancer, there is a very small chance that the specialist will not see a cancer during the procedure. This may happen in around 1 out of every 20 people (5%). This can happen because the bowel wasn't completely empty, or the colonoscopist wasn't able to pass the colonoscope through the whole length of the bowel, or rarely, because the specialist missed the cancer during the investigation.

If you are concerned about the results of your colonoscopy, particularly if you have ongoing symptoms, you should discuss this with your GP.

Back to Bowel screening

The bowel

The bowel is part of our digestive system and is made up of the small bowel and the large bowel.

What to do with your FOB test kit

This test checks bowel motions for tiny amount of ‘hidden blood’. It is not a test for cancer, but can indicate whether further tests are needed to examine the bowel.