BMJ Group Medical Reference
This information tells you about a test to look inside your bowel. It explains how the test is done, how good it is, the risks involved, and what to expect afterwards.
The information about the accuracy and risks of the test is based on research studies and may be different in your hospital. You may want to talk about this with the doctors and nurses treating you.
What is a colonoscopy?
A colonoscopy is a test to check for disease inside your colon and rectum. Your colon and rectum make up the lower half of your gut and are often called your bowel.
During a colonoscopy, a doctor puts a tube with a camera on the end (a colonoscope) into your bottom (anus) and passes it up through your rectum and colon. This allows your doctor to look for problems such as cancer, inflammation (red, swollen patches), or polyps (small lumps, like cherries, on the inside wall of your bowel).
If your doctor sees anything unusual during the test, he or she can take samples of tissue to look at more closely. This is called a biopsy.
Why do I need this test?
You might need a colonoscopy if you have symptoms that could be caused by cancer of the colon or rectum ( bowel cancer). These symptoms are:
Blood in your stool when you go to the toilet (you might notice blood in the toilet bowl, on the toilet paper, or covering the stool)
A change in your bowel habits, such as having bowel movements more often each day or diarrhoea.
But most people with these symptoms don't need a colonoscopy and don't have cancer. The National Institute for Health and Care Excellence (NICE), the organisation that advises doctors about which treatments should be available on the NHS, says that you may need a colonoscopy if any of the following happen: 
You're over 40 and you've had both of the main symptoms (blood in your stools and a change in bowel habits) for at least six weeks
You're over 60 and you've had one or both of these symptoms for at least six weeks. This is because older people have a higher risk of cancer. If you have bleeding, your doctor will check that you don't have any itching around your anus. Itching or soreness around your anus is more likely to be caused by piles than cancer. Piles are swollen veins just inside your anus
You or your doctor can feel a lump in your abdomen, or your doctor can feel a lump in your rectum when he or she examines you internally
You've had polyps before, or have a family history of bowel cancer
A blood test shows that you have anaemia. Anaemia means you have a shortage of iron in your blood. It makes you feel tired. It can be a sign that you are slowly losing blood from your bowel, but not enough to see in your stools
You've had another test, such as a barium enema, that shows you have a problem in your bowel.