Expert reviewer, Mrs Sara Badvie, Consultant Laparoscopic, Colorectal and General Surgeon
Next review due, November 2022

A colonoscopy is a procedure that uses a narrow, flexible, telescopic camera called a colonoscope to look inside your large bowel.

It can be used to check your large bowel for signs of bowel cancer, and to investigate symptoms affecting your bowel.

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Why do I need a colonoscopy?


You may be offered a colonoscopy as part of the national bowel cancer screening programme in England. Bowel cancer screening is offered to everyone over the age of 60, every two years.

The first stage of the screening programme is a kit that you complete at home, called the faecal immunochemical test (FIT). You use this to collect a sample of poo, which is then tested for traces of blood that you wouldn’t usually notice by eye. If blood is found in your poo, you’ll be offered a colonoscopy. This happens to about two in every 100 people who take the FIT test.

Other reasons for a colonoscopy

There are several other reasons why your doctor may recommend you have a colonoscopy. These include the following.

  • Investigating symptoms that could indicate bowel cancer, such as bleeding from your bottom or a change in your bowel habits (for example, needing to poo more often, diarrhoea or constipation).
  • You have a strong family history of bowel cancer (for example, two or more first-degree relatives have had bowel cancer, or one relative under the age of 45 has had it).
  • Investigating symptoms of inflammatory bowel disease (for example, ulcerative colitis or Crohn’s disease), and monitoring your bowel if you’ve been diagnosed with these conditions.
  • Monitoring your bowel if you’ve previously had a polyp or cancer removed, or if you have a family history of a genetic polyp syndrome.
  • Following another test – such as a CT scan or virtual colonography, if you need further assessment or treatment.

Your doctor or nurse can also perform a biopsy (remove a sample of tissue) or remove polyps during a colonoscopy if necessary.

Giving your consent

A specially-trained nurse or doctor, called an endoscopist, will carry out your colonoscopy. You’ll meet them beforehand, so they can explain exactly what will happen during the procedure, what to expect afterwards and the potential complications. They will also assess whether you’re fit enough to have a colonoscopy – tell them if you’re pregnant, breastfeeding or have any health conditions. If you’re invited for a colonoscopy as part of the screening programme, you’ll have an appointment with a specialised screening practitioner to go through everything with you.

Be sure to ask any questions you have and ask for more time to make a decision if you need it. If you’re happy to go ahead with the procedure, you’ll be asked to sign a consent form – so it’s important to make sure you feel properly informed.

Some units where colonoscopy is carried out are JAG accredited. JAG stands for the Joint Advisory Group on Gastrointestinal Endoscopy. A unit with JAG accreditation will have been assessed as having high quality services that meet all the JAG criteria for performing endoscopy procedures.

What are the alternatives to a colonoscopy?

A colonoscopy is usually the best investigation to see inside your bowel, as it can examine the whole of your large bowel, and pick up on very small changes. But it isn’t appropriate for everyone so your doctor might suggest other tests. Other options include the following.

  • Virtual colonoscopy. This involves having a CT scan of your large bowel after gas has been passed into your bowel to expand it.
  • Flexible sigmoidoscopy. This is similar to a colonoscopy but uses a shorter instrument to look inside the lower part of your large bowel only.
  • Barium enema. This involves taking X-ray images of your large bowel, using a contrast or dye (barium) to show up the detail and give clearer X-ray images.

If you have one of these tests, you may need a colonoscopy afterwards to take a closer look at your bowel, or to have a further procedure.

Preparing for a colonoscopy

You’ll usually have a colonoscopy in the endoscopy unit at a hospital and be able to go home the same day. You’ll need to arrange for someone to drive you home, as you’ll usually have a sedative. This can make you feel drowsy for a while afterwards, so you won’t be able to drive yourself until the day after.

Your hospital will give you detailed instructions about what else you need to do to prepare for your colonoscopy.

Medicines to stop

There are certain medicines that you’ll need to stop before having a colonoscopy. These include:

  • iron tablets – you’ll need to stop taking these one week before your colonoscopy
  • anti-diarrhoea medicines, or any medicines that may cause constipation – you’ll need to stop taking these one to two days beforehand
  • medicines that affect the way your blood clots (eg warfarin, aspirin or clopidogrel)

If you take any of these medicines (or any others), let your hospital know before you have the colonoscopy. They can check if you need to stop taking them, and for how long.

Clearing your bowel

Your bowel will need to be completely empty for the procedure so that your endoscopist can see everything clearly. Your hospital will give you a strong laxative that you’ll need to take before your colonoscopy. It usually comes as a powder that you mix with water. You may not be able to take the laxative if you’re pregnant or breastfeeding, so be sure to tell the hospital if this applies to you.

The laxative can quickly give you diarrhoea, so make sure you stay close to a toilet after you take it. You might feel some discomfort or bloating in your tummy (abdomen), and it can taste unpleasant. Try putting the mixture in the fridge before you drink it, as it can make it taste better. Remember that although taking a laxative can be unpleasant, it’s really important to be able to do the colonoscopy effectively.

Your hospital will also give you advice about what you can eat or drink before the procedure. You may be asked to eat only low-fibre foods, or to fast, for at least a day beforehand. This can vary between hospitals, so it’s important to follow the instructions you’re given, and ask if you’re not sure about anything. It’s important to keep drinking plenty of clear fluids though like water, squash or black tea or coffee, to make sure you don’t get dehydrated.

If you take the contraceptive pill, taking laxatives will prevent it being absorbed. Follow the instructions in the patient information leaflet for your contraceptive pills – you may need to take additional contraceptive measures. If you have diabetes, or if you’re having another procedure, such as a gastroscopy, on the same day, the preparation might be different for you. Follow the instructions given to you by your hospital, and if you’re unsure, ask.

At the hospital

When you arrive at the hospital you’ll be seen by a nurse or doctor and will be able to ask any final questions you may have. You’ll be given a hospital gown to change into, and sometimes a pair of paper shorts to wear.

What happens during a colonoscopy?

A colonoscopy usually takes around 30 to 45 minutes, although the appointment itself will last longer than this.

You won’t need an anaesthetic for a colonoscopy, but as the procedure can be uncomfortable, you’ll be offered a painkiller and sedation. A sedative will make you feel more relaxed and a bit drowsy, but you’ll remain awake during the procedure. It’s given as an injection into a vein in your hand at the start of the procedure. At some hospitals, you might be offered gas and air (Entonox) for pain relief instead of a sedative.

Your endoscopist (doctor or nurse) will ask you to lie on your left-hand side and bend your knees. First, they’ll gently examine your back passage using their finger. They’ll then carefully insert the colonoscope, using lubricating gel to make it as comfortable as possible.

Your endoscopist will pump air into your bowel through the colonoscope to inflate it slightly and give a better view of your bowel. This can make you feel a little bloated. Your endoscopist will gently guide the colonoscope through your bowel.

Image showing where a colonoscope is passed

The images from the camera at the end of the colonoscope will appear on a monitor. Your endoscopist might ask you to change your position during the procedure, to help them to look at different areas of your bowel. They may ask an assistant to press on your tummy to help move the colonoscope in the right direction.

Your endoscopist may take a small sample of cells (a biopsy) if needed, and remove any growths (polyps) they find. This is done by passing instruments through the colonoscope. For more information, see our FAQ: What are polyps and should they be removed?

If you find the procedure particularly uncomfortable at any stage, tell your endoscopist and they will stop or change what they’re doing.

What to expect afterwards

If you’ve had sedation for your colonoscopy, you’ll need to be monitored for an hour or so until the effects of the sedative have passed. After that, you should be fine to go home when you feel ready. You’ll need to have someone who can take you home and stay with you overnight while the sedative wears off.

Before you leave, your doctor or nurse may talk to you about how the procedure went and whether they removed any polyps or biopsies. You may be given a copy of the colonoscopy report. If you’ve had a biopsy or polyps removed, you’ll be told how and when you can expect to receive the results.

Recovering from a colonoscopy

You may feel bloated and have some cramping pain after a colonoscopy. This is caused by the gas used during the procedure and usually settles down within a few hours. If you need pain relief, you can take over-the-counter medicines, such as paracetamol or ibuprofen. You may also bleed a little from your back passage if you’ve had a biopsy or a polyp removed. This should stop on its own within a couple of days.

Having sedation can really take it out of you. You might find that you’re not as co-ordinated as usual or that it's difficult to think clearly. This should pass within 24 hours. In the meantime, don't drive, drink alcohol, operate machinery or make any important decisions.

Complications of a colonoscopy

Complications are when problems occur during or after the procedure. The main complications of a colonoscopy are listed below.

  • Breathing problems. You may have a reaction to the sedation, which can affect your breathing or your heart rate. But you’ll be monitored throughout the procedure and given treatment if this happens.
  • A tear in your bowel (bowel perforation), caused by the colonoscope. This is rare, but if this does happen, it’s likely you’ll need an operation to repair it.
  • Bleeding. This is most common after having biopsies or polyps removed, and usually stops on its own. In rare cases though it can become more serious and you may need a blood transfusion.

Contact the hospital department where you had your procedure if you continue to bleed from your back passage or have severe pain in your tummy that doesn’t get better within a couple of days.

Frequently asked questions

  • Polyps are small lumps that can develop in the lining of your bowel. They’re quite common and usually don’t cause any problems.

    But there’s a small chance that over time a type of polyp called an adenoma will grow into a cancer. Most bowel cancers are thought to start as a polyp. So, if your doctor finds any polyps during your colonoscopy, they’ll want to remove them.

    After you’ve had a polyp removed, you might need to have regular colonoscopies to check whether you’ve developed any more. How often you have these will depend on several different factors, including your age, how many polyps you’ve had and when the last one was found.

  • Colonoscopes are thoroughly disinfected with detergents and chemicals before being used again. This means that any bacteria, viruses and fungi are killed and destroyed.

    Colonoscopes are cleaned in dedicated decontamination rooms either at the hospital where they’re used, or in specialist centres away from the hospital. Decontamination involves a strict process. First, they’re manually cleaned, before being disinfected in a machine called an endoscope washer-disinfector. Finally, they’re dried using a special process and stored safely for reuse. The process is monitored and carefully logged. This makes sure that colonoscopes are cleaned thoroughly without any chance of contamination. For example, used colonoscopes enter the decontamination room through one entrance and the clean (reprocessed) ones leave through another, so they never cross paths.

    This is all done by trained professionals who are qualified to clean colonoscopes to the required standards.

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  • Reviewed by Pippa Coulter, Freelance Health Editor, November 2019
    Expert reviewer Mrs Sara Badvie, Consultant Laparoscopic, Colorectal and General Surgeon
    Next review due November 2022