A colonoscopy is an examination of the entire large bowel using a device called a colonoscope, which is like a sigmoidoscope but a bit longer.  The camera relays images to a monitor, which allows the doctor to check for any abnormal areas within the rectum or bowel that could be the result of cancer.

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The entire large bowel needs to be empty when a colonoscopy is performed. It is therefore necessary to have a special diet for a few days beforehand and take a strong laxative (medication to help empty the bowel) on the morning of or day before the examination depending on what time the test is being taken.

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An injectable sedative is given at the start of the test but sometimes Entonox  gas by a mouth piece might be offered during the examination instead. During the test the colonoscope is inserted in to your back passage and slowly moved up along the entire length of the large bowel. This is not usually painful, but can feel uncomfortable. It is necessary for the patient to lie on their left side at the start of the test and change sides during the test to help the camera get around bends in the bowels and straighten out any loops in the colonoscope.

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A colonoscopy usually takes about 20-30 minutes to complete, and most people can go home once they have recovered from the effects of the sedative. If polyps need removing the test might take longer depending on the type of polyp being removed.

After the procedure, it is necessary to recover from the effects of the sedative before being allowed to go home. Transport arrangements will be required as it is not legal to drive for 24 hours after a sedative. It is best for elderly people to have someone with them for 24 hours after the test.

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In a small number of people, it may not be possible to pass the colonoscope completely around the bowel and alternatives to the test might be necessary such as CT colonography.  A colonoscopy is however considered as the gold standard for examination of the large bowel.