What is a colonoscopy?

A colonoscopy is a procedure to look at the inside of your large bowel (colon) using a flexible telescope. A colonoscopy also allows examination of the very end of the small bowel (terminal ileum).

Your doctor has recommended you have a colonoscopy. However, it is your decision to go ahead with the procedure or not. This information about the benefits and risks will help you to make an informed decision. If you have any questions ask your doctor or the healthcare team.

What are the benefits of a colonoscopy?

Your doctor is concerned that you may have a problem in your large bowel (colon). A colonoscopy is a good way of finding out if there is a problem. If the endoscopist (the person doing the colonoscopy) finds a problem, they will be able to perform biopsies (removing small pieces of tissue) to help make the diagnosis.

Are there any alternatives to a colonoscopy?

A CT scan is a less invasive method to examine the colon and doesn’t usually involve preparation with laxatives. A CT scan may therefore be a sensible alternative option in elderly patients in whom bowel preparation is more inconvenient and hazardous.  It also provides additional diagnostic information about other abdominal organs. However a CT scan is a less sensitive test compared to colonoscopy and involves exposure to radiation. Furthermore if a CT scan identifies a possible problem in the colon, a flexible sigmoidoscopy or colonoscopy may still be needed to biopsy or remove the problem.

Colonoscopy procedure diagram


What does a colonoscopy involve?

  • Before the procedure

If you take iron tablets, stop taking them at least a week before the procedure.

If you take warfarin, clopidogrel or other blood-thinning medication, let the endoscopist know at least seven days before the procedure. You will need to follow a special diet and you will be given some laxatives to take the day before the procedure. This is to make sure your bowel is empty so the endoscopist can have a clear view. Follow the instructions carefully. If you have diabetes and are given laxatives, let the healthcare team know as soon as possible. You will need special advice depending on the treatment you receive for your diabetes. If you get severe abdominal pain, contact the endoscopy department or your doctor. The procedure may involve injecting you with a medication (Buscopan) to relax your bowel and make the procedure more comfortable. Buscopan can affect the pressure in your eyes, so let the endoscopist know if you have glaucoma.

The healthcare team will carry out a number of checks to make sure you have the procedure you came in for. You can help by confirming to the endoscopist and the healthcare team your name and the procedure you are having. The healthcare team will ask you to sign the consent form once you have read this document and they have answered your questions


  • In the endoscopy room

Although the procedure is uncomfortable, it should not be too painful. A sedative will be administered through a small needle in your arm or the back of your hand. Alternatively you may be offered a mixture of oxygen and a gas called nitrous oxide (a painkiller and weak anaesthetic) that you breathe through a mask or mouthpiece.

The endoscopist will ask you to lie on your left side.

The healthcare team will monitor your oxygen levels and heart rate using a finger or toe clip. If you need oxygen, they will give it to you through a mask or small tube in your nostrils.

If at any time you want the procedure to stop, tell the endoscopist. The endoscopist will end the procedure as soon as it is safe to do so.

The endoscopist will place a flexible telescope into your back passage. Air will be blown into your large bowel to help the endoscopist have a clear view.

The endoscopist will usually examine the entire colon and may also examine the end of the small intestine (terminal ileum). The endoscopist will be able to look for problems such as inflammation or polyps. They will be able to perform biopsies and take photographs to help make a diagnosis. If they find a polyp, it may be possible to remove it during the procedure.


What complications can happen?

The healthcare team will try to make the procedure as safe as possible but complications can happen. Some of these can be serious and can even cause death (risk: 1 in 15,000). The possible complications of a flexible sigmoidoscopy are listed below. Any numbers which relate to risk are from studies of people who have had this procedure. Your doctor may be able to tell you if the risk of a complication is higher or lower for you.

  • Breathing difficulties or heart irregularities, as a result of reacting to the sedative or your bowel being stretched. If you were given a sedative, your oxygen levels and heart rate will be monitored. Rarely, a heart attack (where part of the heart muscle dies) or stroke (loss of brain function resulting from an interruption of the blood supply to your brain) can happen if you have serious medical problems.
  • Allergic reaction to the equipment, materials or medication. The healthcare team is trained to detect and treat any reactions that might happen. Let the endoscopist know if you have any allergies, or if you have reacted to any medication or tests in the past.
  • Infection. It is possible to get an infection from the equipment used, or if bacteria enter your blood. The equipment is sterile so the risk is low but let the endoscopist know if you have a heart abnormality or a weak immune system. You may need treatment with antibiotics. Let your doctor know if you get a high temperature or feel unwell.
  • Making a hole in your colon (risk: less than 1 in 1,000). The risk is higher if a polyp is removed (risk: less than 3 in 1,000). This is a serious complication. You may need surgery which can involve forming a stoma (your bowel opening onto your skin).
  • Bleeding from a biopsy site or from minor damage caused by the telescope (risk: less than 1 in 1,000). This usually stops on its own.
  • Bleeding, if a polyp is removed (risk: 2 in 100). Bleeding usually stops soon after a polyp is removed. Sometimes bleeding can happen up to two weeks after the procedure. If you take blood-thinning medication and have a polyp, the endoscopist will usually not remove it.
  • Incomplete procedure caused by a technical difficulty, blockage in your large bowel, complications during the procedure, or discomfort. Your doctor may recommend another colonoscopy or a different test such as a CT scan. You should discuss these possible complications with your doctor if there is anything you do not understand.


How soon will I recover?

If you were given a sedative, you will first be transferred to the recovery area where you can rest and have a drink. You will usually recover in about an hour but this depends on how much sedative you were given. You may feel a bit bloated for a few hours but this will pass. If you were given a sedative, a responsible adult should take you home in a car or taxi and stay with you for at least 12 hours. Be near a telephone in case of an emergency.

Do not drive, operate machinery or do any potentially dangerous activities (this includes cooking!) for 24 hours and not until you have fully recovered feeling, movement and co-ordination. You should also not sign legal documents or drink alcohol for at least 24 hours. You should be able to return to work the next day unless you are told otherwise.

The healthcare team will tell you what was found during the colonoscopy and discuss with you any treatment or follow-up you need. Results from biopsies will not be available for a few days so the healthcare team may arrange for you to come back to the clinic for these results. Once at home, if you get pain in your abdomen, significant or continued bleeding from your back passage, or a high temperature, contact the endoscopy unit or your GP. In an emergency, call an ambulance or go immediately to your nearest Emergency department.

  • Lifestyle changes

If you smoke, stopping smoking will improve your long-term health.

Try to maintain a healthy weight. You have a higher risk of developing complications if you are overweight.

Regular exercise should improve your long-term health. Before you start exercising, ask the healthcare team or your GP for advice.


A colonoscopy is usually a safe and effective way of finding out if there is a problem with your digestive system. However, complications can happen. You need to know about them to help you to make an informed decision about the procedure. Knowing about them will also help to detect and treat any problems early.


Author: Mr Simon Parsons DM FRCS (Gen. Surg.) and Mr Jonathan Lund DM FRCS (Gen. Surg.)
Illustrations: Medical Illustration Copyright © Medical-Artist.com

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