About colonoscopy

A colonoscopy is a test that allows your doctor to look inside your large bowel. It’s carried out using a long, narrow, flexible, telescopic camera called a colonoscope. Your doctor will pass the colonoscope into your bowel through your anus (back passage).

A colonoscope can help your doctor to find out what is causing symptoms such as:

  • persistent diarrhoea or a change in your bowel habit
  • bleeding from your rectum or blood in your faeces
  • passing slime or mucus from your bowel
  • feeling tired, weak or breathless

These may be the result of:

Sometimes, a colonoscopy is used to confirm the results of other tests, such as a barium enema. You may also be asked to have a colonoscopy if you have had an abnormal faecal occult blood (FOB) test as part of routine bowel cancer screening. If you’re at an increased risk of developing bowel cancer or polyps, you may have a colonoscopy to screen for these conditions even if you have no symptoms. See our frequently asked questions for more information.

During the procedure, your doctor may take a biopsy (sample of tissue) for examination in a laboratory. He or she may also remove any polyps that are growing on your bowel wall. Polyps are usually benign (not cancerous), but they can change into cancer over a number of years. For more information, see our frequently asked questions.

What are the alternatives to a colonoscopy?

A colonoscopy is usually the best investigation for showing the inside of your bowel. However, it isn’t appropriate for everyone. If you can’t have a colonoscopy, your doctor may suggest other tests. The main alternatives are listed below. 

  • Barium enema. A fluid containing barium (a substance which shows up on X-rays) is put into your bowel via your anus. X-ray images of your abdomen (tummy) are taken which show the inside of your bowel.
  • Virtual colonoscopy. This test involves having a CT scan of your abdomen after gas has been pumped into your colon to expand it. A CT scan uses X-rays to produce three-dimensional images of your large bowel and rectum.
  • Flexible sigmoidoscopy. This is a similar procedure to a colonoscopy but it uses a shorter instrument to look inside your rectum and the lower part of your large bowel.

Preparing for a colonoscopy

Colonoscopy is carried out in hospital and is usually done as a day case. This means you have the test and go home the same day.

Your hospital will write to you telling you what you need to do before your colonoscopy. It’s very important that your bowel is completely empty during the test so that your doctor can see it clearly. The main thing that you need to do to prepare for this is to take a strong laxative – these will be provided for you by the hospital. Usually you will need to take them two days before your test but check this in advance with your doctor or nurse.

The laxative will be quite powerful and will give you diarrhoea, so you will need to stay close to a toilet and drink plenty of clear fluids to make sure you don’t get dehydrated. Clear fluids are those that don’t contain milk such as water, squash and tea and coffee (without milk) – see our frequently asked questions for more information. You may also feel some pain in your abdomen but most people don’t have this. It’s possible that you will find the taste of the laxative unpleasant.

Other things that you’re likely to be asked to do before your colonoscopy include:

  • stopping taking any iron tablets – these make the inside of your bowel look black and cause constipation, which makes it hard for your doctor to see clearly
  • changing your diet in the two days before your test – this may involve reducing how much fibre you eat

If you usually take medicines, for example tablets for high blood pressure, take them as usual, unless your doctor tells you not to. He or she may ask you to stop taking some of your usual medicines if they are likely to make you constipated. If you take medicines that affect the way your blood clots, such as warfarin, aspirin or clopidogrel, tell the outpatient department staff when you receive your appointment letter. The staff will tell you whether you need to stop these before the test.

If you have diabetes and you’re taking insulin or medicines to treat it, contact the outpatient department to tell them. They may be able to arrange for you to be the first to have your colonoscopy on the day of your appointment. You will be given specific information about when to take your medicines or insulin and what to eat before the procedure.

Your doctor will discuss with you what will happen before, during and after your procedure, and any pain you might have. This is your opportunity to understand what will happen, and you can help yourself by preparing questions to ask about the risks, benefits and any alternatives to the procedure. This will help you to be informed, so you can give your consent for the procedure to go ahead, which you may be asked to do by signing a consent form.

What happens during a colonoscopy

The procedure usually takes 30 to 45 minutes to complete.

Your nurse will ask you to put on a hospital gown that opens at the back or a gown and trousers that have a hole cut in the back.

You will be asked to wear an oxygen mask or have small tubes placed just under your nostrils through which oxygen is passed. You will have a drip inserted into a vein in the back of your hand to give you any medicines you may need. You will probably be given a sedative and a painkiller to make sure you're relaxed and comfortable during the procedure.

Your doctor will ask you to lie on your left-hand side and he or she will gently insert the colonoscope into your rectum. Lubricating jelly will be used to make this as easy as possible.

Air will be pumped into your bowel to make it expand slightly, which makes the colonoscope pass through more easily. This might feel uncomfortable and you may pass some wind. Your doctor will gently move the colonoscope up through your bowel. The colonoscope is flexible and the doctor can rotate the tip of the instrument so that it follows the curves in your bowel.

The images from the camera at the end of the colonoscope appear on a monitor – these enable your doctor to look at the inside of your bowel. During the procedure you may be asked to change your position – for example turning from your side on to your back. This helps your doctor to look at different areas of your bowel. He or she may press on your abdomen to help move the colonoscope in the right direction.

During the procedure your doctor may take a biopsy or remove small polyps. This is quick and painless and is done by passing instruments inside the colonoscope. The samples will be sent to a laboratory for testing to see if the cells are benign or malignant (cancerous).

Illustration showing where a colonoscope is passed

What to expect afterwards

You will need to rest until the effects of the sedative have passed. You will usually be able to go home when you feel ready but you will need to arrange for someone to drive you home as you’re likely to still feel drowsy after the sedative. Try to have a friend or relative stay with you for the first 12 hours after your colonoscopy.

At the hospital after your colonoscopy, your doctor may discuss other findings from the test with you before you leave, or you may be given a date for a follow-up appointment. If you have had a biopsy or had polyps removed, your results will be sent to the doctor who referred you for the colonoscopy.

Recovering from a colonoscopy

If you need pain relief, you can take over-the-counter medicines, such as paracetamol or ibuprofen. Always read the patient information that comes with your medicine and if you have any questions, ask your pharmacist for advice.

Sedation temporarily affects your co-ordination and reasoning skills, so you must not drive, drink alcohol, operate machinery or sign legal documents for 24 hours afterwards. If you’re in any doubt about driving, contact your motor insurer so that you’re aware of their recommendations, and always follow your doctor’s advice.

Most people have no problems after a colonoscopy, but you should contact your GP or the hospital department if you:

  • have heavy bleeding from your back passage
  • have a swollen abdomen and abdominal pain that gets worse
  • develop breathlessness or shoulder pain
  • feel unwell and develop a fever (high temperature)

What are the risks?

As with every procedure, there are some risks associated with colonoscopy. We have not included the chance of these happening as they are specific to you and differ for every person. Ask your doctor to explain how these risks apply to you. 


These are the unwanted but mostly temporary effects you may get after having the procedure.

After having a colonoscopy, you may feel bloated and uncomfortable for a few hours as a result of trapped wind. You may find that lying on your front can help. You may also bleed a little from your back passage, especially if you have had a biopsy or polyp removed.


Complications are when problems occur during or after the procedure. The main ones related to colonoscopy are listed below.

  • Your doctor may not be able to see all of your bowel. This can happen if your bowel isn’t empty or the colonoscope can’t be passed round a curve. If this happens, you may need to have the colonoscopy done again or have other tests.
  • You may have a reaction to the sedation, which can affect your breathing or your heart. You will be monitored throughout the procedure and treated quickly if this develops.
  • The colonoscope and the other instruments used during the procedure may damage or tear your bowel. If this happens, you may need an operation to repair it.
  • You may have heavy bleeding if you have had biopsies or polyps removed.

Produced by Polly Kerr, Bupa Health Information Team, February 2013.

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