Colonoscopy is a useful technique, which allows the surgeon to examine the inside of the large intestine (colon). It can be used to investigate rectal bleeding, anaemia or alteration in bowel habit; to examine parts of the bowel not clearly seen on a CT scan; to screen for polyps in patients with a family history of bowel cancer; or as part of follow-up where a polyp or cancer has been removed previously. Removing polyps has been proven to have a dramatic effect in reducing the risk of bowel cancer. The colonoscope is a flexible telescope, which is passed through the anus and rectum into the colon. In addition to direct visual inspection, small samples (biopsies) can be removed and sent for microscopic analysis.

Bowel preparation for a colonoscopy

In order to examine the colon it needs to be empty. This means that you have to take a powerful laxative prior to the examination. Instructions for taking the laxatives will be given to you. The timing depends on whether your examination is scheduled for the morning or afternoon. If you are diabetic or on any kind of anticoagulation then you will be given specific instructions about measures to take prior to the examination.

The day of the procedure

You will be admitted to the hospital and be asked to sign a consent form to the procedure if you have not already done so in clinic. You can drink right up to the procedure and take your normal tablets; though if you have a long drive you might wish to stop drinking in advance of that to avoid the need to stop on the way.


Sedation helps to minimise discomfort and anxiety about colonoscopy but is not essential and the choice is yours. If you opt to have sedation, a small intravenous cannula is inserted to give you the two drugs used, midazolam and fentanyl, which induce drowsiness and provide pain relief. Midazolam also has an amnesic effect, so you may not have full recollection of the procedure. As you are sedated you will be given additional oxygen through the nose which is routine practice.

It may be possible to perform the procedure with Entonox (nitrous oxide gas, sometimes called laughing gas) which you can breathe in during the colonoscopy as you wish. This gives very good pain relief without sedation. Some patients prefer to have no sedation at all and then can drive etc immediately afterwards. 

You can discuss these options with your Consultant.  

The Procedure

The colonoscope is inserted and gently manipulated along the bowel. This can be aided by moving your position at times and occasionally some pressure can be applied to the tummy to help. It can take 10-45 minutes to perform but you will be away from the ward longer than this as after you will spend some time in the recovery area.

The images will appear on a screen, which you can watch if you like. Any abnormality can be photographed and some biopsy samples can be taken. If there are bowel polyps then these can usually be removed during the procedure.


Following the procedure you will be returned to your room on the ward where the effects of the sedative will begin to wear off. Your surgeon will see you and explain the outcome of the examination. You may not recall all that is said because of the continuing effect of the sedative so it is useful to have a companion who can explain the findings later. The sedative however will affect your judgment so it is essential that your companion drive you home. You may get some abdominal colic as the gas used to inspect the bowel is passed but this will wear off quickly. You should not drive a car or operate machinery for 24 hours. The results of any biopsies sent for pathology analysis take a week or so to come through and these can be discussed when you are reviewed in clinic or will be sent to you in the post/via email.


Colonoscopy is a very safe procedure with few risks, but complications do occur rarely. The two main dangers are perforation and significant bleeding; the risk of these is highest after removal of a large polyp. Fortunately these are extremely rare – the risk of perforation is about 1 in a thousand and bleeding about 1 in two hundred; when they do occur, hospital admission is necessary and sometimes an operation is required to correct the abnormality. In our practice, no patients have required surgery despite performing many thousands of procedures.

Occasionally it is not possible to see all the way around the colon with the instrument because of the configuration of the bowel. Under these circumstances a CT scan may be required to look at the remaining colon.