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A colonoscopy is a procedure to look at the lining of your large bowel (colon). The procedure involves the insertion of a narrow and flexible telescopic camera, called a colonoscope into the back passage (anus). Colonoscopy can find inflamed tissue, ulcers and abnormal growths (called polyps). It can be used to look for signs of colorectal cancer and help your team to find the cause of changes in bowel habit, abdominal pain, bleeding from the back passage and weight loss.

Sometimes we will take tissue samples which are sent off to be looked at under a microscope. It is also possible to perform procedures at the time of the colonoscopy, such as removing an abnormal growth. The colonoscope can also be used to stretch open (dilate) parts of the bowel which are narrowed.


A colonoscopy is usually done under sedation (medication to make you sleepy) which will relax and make you more comfortable, but it does put you to sleep. Some people prefer to have it without sedation, so they are wide awake during the test. This means you do not need to arrange someone to take you home and stay with you after the test. We can give you pain relief with an inhaled painkiller called Entonox®. This is the same as the ‘gas and air’ that women can use when in labour. Gas is put into the colon so there is a clear view of the lining. You will need to make changes to your diet and take medication before the procedure so that the colon is empty at the time of the procedure. A routine colonoscopy takes around 45 minutes. Please ask a member of staff if you have any questions about your procedure.

One of the alternatives to colonoscopy is a special scan called CT colonography. This is an X-ray examination of the bowel, and although it is less invasive than a colonoscopy it requires a short, small tube to be inserted inside your back passage to blow air in. There is a very low risk of causing damage to the bowel.

Some patients may be able to have a procedure called a capsule endoscopy which involves swallowing a tiny camera which is the size of a large vitamin pill. It takes pictures as it travels through the bowel, and sends them to a recording device you wear on your waist. The only risk is that the capsule may get stuck in the bowel. The risk of this happening is very low but you will need treatment to remove the capsule if it gets stuck.

If we are unable to complete your colonoscopy on the day we will try to offer you one of these alternatives to avoid you having to come back on a different day.

Colonoscopy is a routine procedure with few side effects or complications. However you should be aware of the following complications:

Bowel perforation – the colonoscope can sometimes cause a small hole (perforation) in your large bowel. This happens in around one in every one thousand (1 in 1,000) colonoscopies. If this happens, you will need a surgical procedure in operating theatres to repair the perforation.

Bleeding – if you have had polyps removed or biopsies there is a small risk of bleeding in around one in every one hundred and fifty (1 in 150) procedures. This can happen up to one week after the procedure and usually stops by itself. A small number of patients who experience bleeding need to be admitted to hospital for observation.

Incomplete test – in less than ten per cent (1 in 10) of procedures it is not possible to complete the test. This may be because the colon is not empty enough to be able to see the lining. Sometimes the bowel loops and twists which makes the procedure painful. A small number of patients find the procedure too uncomfortable so it is not possible to finish it. You may then need one of the alternative tests described in the section above ('What alternatives are available').

Missed lesion – although colonoscopy is the best procedure to help diagnose many problems, no procedure is perfect. There is a small risk that we might miss a lesion or other important finding during your procedure.

Side effects of sedation – there is a small risk that the medication we use to sedate you can cause heart problems or difficulty breathing. We will monitor you during the procedure and we can give you medication to reverse the effects of the sedation if we need to.

Colonoscopy may be painful as the scope moves around the corners in the bowel. Usually if it is sore, the discomfort will only last a minute or two as the camera gets past the corner. If it is uncomfortable please let us know during the procedure as there are a number of things we can do to help your comfort.

Bowel preparation for a colonoscopy is undertaken to clear the colon of faeces in order to ensure a clear view of the lining of the bowel during the procedure.

It is essential that you read and follow the instructions prior to your procedure. The success of this procedure depends on your colon being as empty as possible and if the colon is not adequately cleansed the procedure may have to be repeated. If you are unable to complete the preparation as instructed then please check with us at the Endoscopy Recovery Unit before coming.

There are different things to do as soon as you get your appointment, seven days before the test, three days before the test, the day before the test and on the day of the test. Please make a note of what needs to be done.

It is unusual for Moviprep® bowel preparation to be harmful but there is a risk of developing dehydration, low blood pressure or kidney problems with this medication.

People who are particularly at risk of dehydration and loss of salts include: frail elderly people, children, and people with renal impairment, congestive cardiac (heart) failure, liver cirrhosis, or taking diuretics (water tablets). Moviprep® is especially designed to be safe in these situations so the risk is very low.

If you are pregnant or think you might be do not take the Moviprep® as it could be very dangerous. Please contact the Endoscopy Unit for advice.

The type and seriousness of side effects differ from person to person. Possible ones are nausea or vomiting, bloating, stomach pain, soreness around the back passage and dehydration (please remember to drink plenty of fluids).

If you have any allergic reactions including a rash, an itch, redness or swelling please seek medical advice.

Tablets and other oral medication should not be taken within one hour of Moviprep® as they may be flushed through too quickly and not absorbed so they will not work properly.

You should follow any specific advice you have been given with regard to your regular medications. Your doctor may ask you to stop medicines such as:

  • diuretics (water tablets)
  • non-steroidal anti-inflammatory drugs (NSAID) such as ibuprofen.

Note: Patients taking immunosuppressant medication for transplanted organs should seek the advice of their transplant doctor before taking Moviprep®, as it might mean the anti-rejection drugs work less well.

Women taking the oral contraceptive pill should take alternative precautions during the seven days following taking Moviprep®, as the pill may not work.

If you are on blood thinning medication – such as warfarin, rivaroxaban or clopidogrel – let us know. The person booking your test may have already given you instructions about what to do but it can be dangerous to stop taking them without expert medical advice. If you continue to take them without telling us we may not be able to do the procedure properly.

If you have diabetes you must contact your Diabetes Specialist Nurse at your hospital or your GP surgery for advice.

Stop taking any iron tables, including multivitamins, but continue with all other medications and laxatives which you usually take. If you are unsure whether to stop any medicines please contact the Endoscopy Unit.

Do not eat any high fibre foods such as wholemeal bread, cereals, fruit and fruit juice, vegetables, nuts, seeds, salad and muesli. Do not eat red meat.

Please avoid these foods, otherwise we will not be able to look at your colon properly and the test will not be as effective.

Foods that can be eaten three days before your colonoscopy:

Cereals: only Rice Krispies® or cornflakes Bread: only white bread or toast.
Eggs: scrambled, fried, poached or boiled Meat: fish or chicken.
Rice: white rice.
Pasta: white pasta.
Potatoes: boiled, mashed, fried or baked - do not eat the skin.
Cheese: any cheese and cheese sauce.
Dairy: butter, margarine, milk and yogurt (plain).
Spreads: seedless jam or marmite.
Vegetarian: you may eat tofu or soya products.
Dessert: ice cream, jelly (plain).


From 14:00 do not eat any food until after your procedure. You may continue to drink plenty of fluids (black tea or coffee, fruit squash, sports energy drinks such as Lucozade® and clear strained soup) but do not drink fruit juice and do not put any milk in your tea or coffee.

From 16:00 open two Moviprep® sachets (one sachet A and one sachet B) and pour them into a large jug. Add one litre of lukewarm water (not chilled) to the jug, and stir until all the powder is dissolved. The water should look clear or slightly hazy.

Drink one glassful (about 250ml) every 30 minutes or so until you have drunk the whole litre. This can take up to two to three hours – there is no need to hurry. You could put it in the fridge to chill it, which will make it easier to take, or you could try sipping it through a straw if you do not like the taste. You must also drink an extra half litre (500ml) of clear fluids (such as water, squash or black tea) in addition to the Moviprep®. This is to ensure you do not become dehydrated and also to ensure all the waste gets flushed out of your bowel.

You should expect diarrhoea to start a few hours after the first dose of Moviprep®. You should also expect some crampy tummy pain. Moviprep® is designed to clean the bowel out so it is sensible to stay near a toilet.

You may find your bottom gets sore from going to the toilet so often. To prevent soreness, you may apply petroleum jelly (such as Vaseline®) or a barrier cream (such as Sudocrem® or Drapolene®) to your bottom after you got to the toilet.

Note: If you develop vomiting, severe diarrhoea with dizziness, headache and confusion, contact the Endoscopy Unit during working hours. At other times please contact the on-call endoscopy registrar via switchboard.

Between 07:00 and 08:00 open the remaining two Moviprep® sachets (one sachet A and one sachet B) and repeat the process again. Remember to drink the entire litre of Moviprep® solution along with a further half litre of clear fluids. Again, this can take up to two to three hours. This will clear out any waste that has accumulated overnight. Your bowel is now ready.

You may continue to drink clear fluids but you must stop drinking two hours before the procedure to ensure your stomach is completely empty. Moviprep® counts as clear fluids.

If you are also having an upper gastrointestinal endoscopy (OGD or gastroscopy) you must stop drinking four hours before.

If you have any queries please contact the Endoscopy Unit.

When you arrive at the Endoscopy Unit, please report to the reception. A nurse will take you to the admissions area where they will check your details and check your blood pressure. We will ask you to change into a hospital gown.

Delays: sometimes due to emergencies and other unforeseen events, your appointment may be delayed. We try our best to see everyone on time but please be patient if your appointment is delayed.

After this you will be meet the member of staff who will perform the colonoscopy (endoscopist) who will obtain your consent.

We want to involve you in all the decisions about your care and treatment. If you decide to go ahead with treatment, by law we must ask for your consent and will ask you to sign a consent form. This confirms that you agree to have the procedure and understand what it involves. Staff will explain all the risks, benefits and alternatives before they ask you to sign a consent form. If you are unsure about any aspect of your proposed treatment, please speak with a senior member of staff again.

Before the procedure starts you will be asked to lie on a trolley. You will have some equipment attached to you so that we can monitor your blood pressure and oxygen levels. You will be given a small amount of oxygen during the procedure.

You will be asked to lie on your left side. However, you may be asked to change your position during the course of the colonoscopy. If you are being sedated the endoscopist will administer the medication via a small needle in the back of your hand at this time. Sedation may take a few minutes to work. It is not an anaesthetic, it is designed to help you relax, not put you to sleep.

If you have had sedation you will be taken to the recovery area. You will stay there for around one hour so that the effects of the sedation can wear off. Your blood pressure and oxygen levels will be monitored during this time and you will also be offered a drink and a snack. If you have not had sedation you will not need to stay in recovery for as long.

We will give you the results of your colonoscopy and some information about signs and symptoms to look out for after you have gone home.

You may eat and drink (non-alcoholic drinks) as normal as soon as you get home. The gas that was inserted into your bowel during the procedure may cause you to feel bloated or full for several days after the procedure. It may take up to a week for your normal bowel habit to return.

  • No solid food should be eaten for at least two hours before taking bowel preparation medication.
  • Bowel preparation medication may cause dehydration so you should maintain good fluid intake whilst taking these medications.
  • If you develop symptoms of dehydration such as dizziness, thirst or reduce urine, please seek medical attention immediately.
  • If you have had sedation you must have someone to take you home and stay with you for 24 hours after the procedure.
  • Do not drink alcohol for 24 hours after having sedation.
  • Do not drive a car or operate heavy machinery for at least 24 hours after having sedation.
  • Do not drive a car or operate heavy machinery for at least thirty minutes after having Entonox®.

If you have any symptoms such as bleeding, black tarry stools or abdominal pain, please contact us immediately.

There are a number of websites which provide more information about having a colonoscopy:

Cancer research UK

Bowel Cancer UK

Crohns and Colitis UK

UCLH cannot accept responsibility for information provided by other organisations.

Endoscopy Booking Team (Monday to Friday 09:00-17:00)
Direct line: 020 3456 7022
Switchboard: 08451 555 000 / 020 3456 7890 ext. 67022

Endoscopy Recovery (Monday to Saturday 09:00-17:00)
Direct line: 020 3447 3282
Switchboard: 08451 555 000 / 020 3456 7890 ext. 73282

On-call Endoscopy Registrar (out of hours emergencies only)
Switchboard: 08451 555 000 / 020 3456 7890

Address: Endoscopy Unit, University College Hospital, 2nd Floor Podium, 235 Euston Road, London, NW1 2BU


Page last updated: 22 May 2024

Review due: 01 April 2025