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A colonoscopy is a routine test to check the lining of your bowel. Having a colonoscopy can feel daunting, and it’s natural to have questions about the procedure and how to prepare. 

This information explains more about having a colonoscopy, including the benefits, risks and any alternatives and what you can expect when you come to hospital.

What is a colonoscopy?

A colonoscopy is a routine test to examine the lining of your bowel, also called the large intestine or colon.

This is done using an a flexible, thin tube which has a camera inside it. This is called an endoscope.

It is passed through your bottom (anus) and carefully moved around the large bowel by a specially trained doctor or nurse called an endoscopist.

Why it's done

By looking down the endoscope, we will be able to get a clear view of the lining of your colon.

This will help us to diagnose your symptoms or check any bowel condition that you have had diagnosed in the past. You may have been advised to have a colonoscopy if you have:

  • bleeding from your bottom
  • pain in your tummy
  • persistent diarrhoea
  • changes to your bowel habits
  • a strong family history of bowel cancer
  • been placed on a bowel cancer screening pathway
  • have an existing condition that needs reviewing such as Crohn’s disease or colitis

Preparing for a colonoscopy

Eating, drinking and bowel preparation

It is very important to follow our instructions about eating and drinking before your colonoscopy.

Please read our diet advice and bowel preparation information.

Full details will be given to you when you come to the endoscopy unit to collect your bowel preparation or in the outpatient department when you are first referred for a colonoscopy.

If you have any questions about the test, you will be able to discuss them at this time.

Tell us about your medicines

Tell your doctor or nurse if you are taking any medicines that thin your blood. This includes antiplatelet medicines (such as aspirin or clopidogrel) or anticoagulant medicines (such as warfarin or rivaroxaban). You might need to stop them temporarily before your procedure.

Also tell your doctor or nurse if you have diabetes as you may need to change the dose of your diabetes medicines, as you will need to reduce your eating and drinking before the procedure.

More information on stopping any medicines will be given to you when you come for pre-assessment.

Please let us know if you are taking any regular medicines (including anything you buy yourself over the counter or any herbal or homeopathic medicines) and if you have any allergies to any medicines.

You should continue to take all of your medicines as normal, unless you have been told otherwise by the doctor or endoscopy nurse.

Preparation checklist

  • If you have not received your bowel preparation please contact the endoscopy department.
  • If you are planning to have sedation, or have been advised to, arrange for a friend or relative (18 years of age or older) to escort you home after your appointment.
  • If you do not organise an escort, or if they are under 18 years old, we will not be able to give you sedation and the procedure may be cancelled.
  • Make a note of the date of your appointment.
  • If you are taking medicines to prevent blood clots please contact us for advice before your appointment.
  • Do not eat anything for 6 hours before your appointment or drink anything for four hours before. You may have small sips of water for up to 2 hours before. Read our information about eating and drinking.
  • Wear loose-fitting clothes on the day of the test.

When you arrive at the endoscopy unit

On arrival, please give your name to the receptionist or nurse.

We have a health questionnaire that we ask you to complete which will help to speed up your admission or pre-assessment. This can be filled in on the day of your admission in reception.

Waiting times

Please be aware that sometimes another patient who arrived after you may be called in before you are. This does not mean you have been forgotten, but that the other person is on a different list to you.

We also deal with emergencies. These can take priority over your appointment, meaning we may have to ask you to wait. We apologise in advance if this happens but please be patient with us and check at the reception desk if you are concerned.

We do everything we can to avoid keeping you waiting any longer than necessary, but because every procedure takes a different length of time to complete, sometimes it’s hard to give exact timings.

We’ll update you regularly as to how long you are likely to be with us. But please be prepared to be with us for the whole morning or afternoon, depending on whether you are a morning or afternoon admission.

Checking your medical history

At check in we will ask you to wait in the waiting area until you are seen by an endoscopy nurse, who will ask you about your medical history.

Please tell the nurse if you have had any reactions or allergies to other examinations in the past.

Getting ready for the procedure

In case there are polyps present, we will ask you to take off all your jewellery before the examination. This is because you should not wear any metal for the technique we use to remove polyps. Because of this, you may wish to leave any valuable jewellery at home, as we cannot be responsible for any valuables lost while in the unit.

You will be asked to remove all of your clothing and change into a gown and some dignity disposable underwear. You may want to bring your dressing gown and slippers with you (we do supply non slip socks).

Meeting the endoscopist 

Once you are ready you will be taken to the second waiting area, signposted ‘sub wait area.’ Your endoscopist will explain more about the procedure and answer any questions you may still have.

During the procedure

The test itself only takes about 30 minutes and an endoscopy nurse will be there throughout to reassure you.

This test is normally performed with sedation or an injection of painkillers but you can choose not to have sedation if you prefer.

Sedation is a medicine that makes you relaxed and sometimes sleepy but does not put you to sleep. Your endoscopist will explain this to you in more detail.

If you are concerned about pain or discomfort you might want to use gas and air (Entonox®). This is commonly used for pain during childbirth. It is also used widely by the ambulance service and in hospitals for a variety of procedures and conditions.

Read more about gas and air (entonox)

You will be asked to lie down on your left-hand side on a couch with your knees bent. The endoscopist will start by inserting a finger into your anus to perform a rectal exam, this will help lubricate the anal passage and identify if there are any significant haemorrhoids or growths able to be felt in your anal passage.

The endoscope will then be inserted into your lower bowel through your bottom (anus). Air will be passed down a channel in the endoscope, expanding your bowel to make it easier to see the lining.

This may make you feel slightly bloated and you may feel the urge to go to the toilet, but you will not be able to do this as your bowel will be empty.

Many people fart during the test. This is perfectly normal and is nothing to be embarrassed about.

If you have polyps, your endoscopist can remove these. You will not feel this. When a polyp is particular big or in a difficult position, you might need to wait for another time for it to be removed and re-booked on a specialised list.


You might have some cramping and discomfort due to the air used to inflate your bowel or brief periods of discomfort particularly when the endoscope is first inserted and when the scope passes around bends in your bowel.

Occasionally the nurse will press gently on your stomach or your position may be altered to aid the passage of the scope.

You will be given painkillers through a needle that will be inserted into a vein to help keep you comfortable. You will not feel anything if any biopsies are taken

Recovering after the procedure

After the procedure you will be taken to the recovery area.

If you have had sedation, you will need to rest quietly until your observations are stable (usually an hour). The nurse will check your blood pressure and pulse and will take you to the discharge area when you are stable.

You can bring your own sandwich or other snacks with you if you are concerned that you will be very hungry otherwise you will be offered some tea and biscuits in the discharge waiting area.

You must arrange for a relative or friend to take you home approximately one hour after the test. This person should be 18 years of age or older.

It is recommended that someone stays with you overnight. You will not be able to drive or operate any machinery for the rest of the day and will need to rest quietly at home.

Please note that your appointment will be cancelled on the day if you wish to have sedation but have not organised an escort home. If you are unable to arrange someone to collect you, please contact us to discuss alternative arrangements.

If you have not had sedation

If you have not had sedation you will be escorted to the discharge area where a nurse will explain your results and give you a copy for your own records.

You will be able to leave as soon as the nurse has given this to you.

Caring for yourself after you leave hospital

The sedation lasts longer than you may think. You must not:

  • drive or ride a bicycle
  • operate machinery or do anything requiring skill or judgement
  • drink alcohol
  • take sleeping tablets
  • go to work
  • make any important decisions, sign contracts or legal documents

You should rest at home following your procedure and should be able to carry out your normal activities 24 hours after the test.

You should go straight to your nearest emergency department (A&E) if you develop severe tummy pain or a fever (high temperature), are being sick, or passing large amounts of blood after the test. You should take your endoscopy report with you.


The doctor or specialist nurse will often be able to tell you your results before you leave the hospital.

If you have had a sedative, it is a good idea to have someone with you when the results are being discussed, as you may not remember all of the details afterwards, due to the sedative.

If you have had biopsies taken the results may take up to 2 weeks.

Follow-up appointments

If you need a follow-up appointment, this will be arranged for the next available clinic or, if it is urgent, you will be given your appointment on the same day.

You might be informed after your procedure that your follow up is a virtual clinic. This means the clinical team reviews your endoscopy and biopsy results without you there to decide what the next steps are based on the results.

You and your GP will then receive an outcome letter from that clinic informing you if you need to return to an outpatient clinic or if you can be discharged back to your GP.

Risks of having a colonoscopy

Serious complications are extremely rare.

The most serious risk is the endoscope damaging your colon during the test. This can cause an infection, bleeding or a perforation (tear) of the lining of the bowel. If this happens, your tummy may become painful and bloated and you may need medicine or surgery to treat the problem.

If a biopsy is taken or a polyp is removed during the test, you may pass a small amount of blood from your bottom after the test. This should only happen up to 12 to 24 hours after the test and is usually no more than a few teaspoons full.

Please note that occasionally we might not be able to complete the test, or do the test at all. This can happen if you find the procedure too uncomfortable or if the bowel preparation did not empty your bowel completely. In this case, the test may need to be repeated or we may suggest a different procedure.

Your doctor or specialist nurse will discuss these potential risks with you in more detail. 

Alternatives to a colonoscopy

  • CT (computerised tomography)/CT enema. This is a special type of X-ray machine that can give more details than normal X-rays. However, it cannot be used to take biopsies or remove polyps, so you may still need a colonoscopy.
  • A sigmoidoscopy. This is similar to a colonoscopy but only looks at the lower part of the bowel. If your doctor or nurse cannot diagnose your symptoms after this test, you may still need a colonoscopy.
  • Faecal occult blood test (FOB). This tests for hidden blood in your stool, but you may still need a colonoscopy if this test is positive.

The tests above are generally considered to be less accurate than a colonoscopy and some of them involve radiation.

Giving your permission (consent)

We want to involve you in decisions about your care and treatment. If you decide to go ahead, you will be asked to sign a consent form. This states that you agree to have the treatment and you understand what it involves. If you would like more information about our consent process, please speak to a member of staff caring for you.

Contact us


If you need to change or cancel your appointment please ring 020 7188 8887.

Questions, problems or concerns

Please contact the endoscopy unit staff for advice (Monday to Saturday 9am to 5pm)

Nurse in charge, phone 020 7188 7188 ext 54059

St Thomas' reception desk, phone 020 7188 7188 ext: 54046 

In an emergency, out of hours

Out of hours is 6pm until 8am the next day, and Saturdays and Sundays.

Phone 020 7188 7188 and ask to be put through to the on-call gastroenterology registrar (via Rota watch).



Ref number: 0261/VER2

Last reviewed: Feb 2019 | Next review: Feb 2022

A list of sources is available on request


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