Colonoscopy diet advice and bowel preparation

A colonoscopy is a test to help find what is causing your bowel symptoms. A specially trained doctor or nurse puts a soft, flexible tube with a camera at the end through your bottom (anus). This tube then passes up inside your bowel.

You can find more information about having a colonoscopy on the NHS website.

This information explains what you need to do in the days before your colonoscopy. Please read the guidance carefully at least 4 days before your appointment. It tells you:

  • what you need to eat and drink
  • how to take bowel preparation solution
  • what medicines you need to stop taking

Your bowel needs to be empty for the colonoscopy. We can then see the lining of the bowel. Diet and a bowel preparation solution help to make sure that your bowel is empty. If you do not follow this guidance, your bowel may not be clear and we may have to repeat your test.

It’s particularly important to follow our guidance if you take medicines. You may need to stop some of them up to 4 days before your colonoscopy.


Please tell our nurses as soon as possible if you:

  • take medicines to thin your blood or prevent it from clotting (anticoagulants or antiplatelets), such as warfarin, rivaroxaban or clopidogrel. This does not apply if you only take aspirin
  • have diabetes and are on insulin
  • have allergies to any medicines
  • are allergic to latex (rubber)

At your pre-assessment appointment, please tell us if you take any regular medicines. This includes:

  • anything you buy from a pharmacy or shop
  • herbal or homeopathic medicines 

Food and drink

You can continue to eat and drink as normal 4 to 3 days before your colonoscopy.

You need to change your diet 2 days before your colonoscopy. Please:

  • stop eating foods that contain fibre (this is found in fruit, vegetables, pulses and grains, such as wholemeal bread and cereals)
  • eat smaller meals
  • drink more fluid

Bowel preparation

You need to take a bowel preparation solution. We give you sachets of strong laxatives. They are used to help you poo and empty your bowel ready for the test.

You need to take the sachets:

Do not follow the instructions on the packet. Instead, please follow the instructions in this information. There is guidance for the specific bowel preparation solution that we have prescribed you. 

Bowel preparation solutions

There are different names for bowel preparation solutions and we give them for different reasons. We prescribe you one of these solutions:

  • CitraFleet® or Picolax® (sodium picosulphate)
  • Klean-Prep®
  • Moviprep®

If we are concerned about your kidneys or you have kidney problems, we will prescribe you Klean-Prep or Moviprep. These are less likely to affect how your kidneys work than CitraFleet or Picolax (our standard bowel preparation solution).

You might need to have a blood test before and after the procedure to check how your kidneys are working. Please tell us if:

  • you cannot drink a large amount of fluid
  • the amount of fluid that you can drink each day is restricted
  • a doctor has told you that your kidneys are not working properly

Senna tablets

We might give you senna tablets as well. Senna is a natural laxative, which is used to treat occasional constipation (difficulty pooing).

To use senna tablets safely:

  • keep them out of children's reach and sight
  • store them in the original container
  • check that the foil is not broken before use. If it is, do not take that tablet
  • do not take them if you know that you are allergic to any of the ingredients
  • do not store them above 25C
  • do not freeze them
  • do not use them after the expiry date

You may get mild stomach pains for a short time. If the pain becomes bad or you have other symptoms after taking the tablets, tell your doctor or pharmacist.

Talk to a doctor or pharmacist if:

  • you have sharp or long-lasting stomach pain
  • your tummy (abdomen) hurts when you touch it or move
  • you accidentally take too many senna tablets
  • your doctor has told you that you have an intolerance to some sugars (find it hard to digest them)

Going to the toilet

You need to stay near a toilet when you start taking the bowel preparation solution.

You can expect to poo frequently and have diarrhoea (watery poo). This starts within 3 hours of taking the first amount (dose) and continues until after your last dose.

We plan when you take the bowel preparation solution carefully. This means that the effects wear off before you are due to travel to the hospital.

Side effects of bowel preparation

It is common to have some bloating (when your tummy feels full and uncomfortable) or stomach cramps.

You can use a barrier cream (a cream to protect the skin from damage or infection), such as zinc and castor oil, on your bottom. This can prevent soreness during your frequent trips to the toilet.

Keep drinking clear fluids to stop yourself from getting dehydrated. As a guide, try to drink about 1 glass every hour. Signs that you are dehydrated include dizziness, headache and confusion.

Please report any allergic reactions, such as a rash, itchiness or redness, to the endoscopy unit or your GP.

Please call the endoscopy unit if:

  • you are sick (vomit) at any time after taking the bowel preparation solution
  • you have any concerns about the bowel preparation solution

Call 020 7188 3221 (St Thomas’ Hospital) or 020 7188 1728 (Guy’s Hospital).

Call 999 if:

you have a reaction that causes:

  • swelling of the face, lips, tongue or throat

If you take the contraceptive pill

If you take the contraceptive pill, it may not work when you have taken the bowel preparation solution. Use another type of contraception (such as condoms) to avoid getting pregnant. Please speak to your GP, pharmacist or family planning clinic for more advice on contraception.

If your health gets worse

We ask you to complete a health questionnaire. This helps to speed up your appointments and pre-assessment. You can fill in the questionnaire on the day of your appointment.

If your health gets worse since last seeing the doctor or specialist nurse in the clinic, please call the endoscopy nurses for advice on 020 7188 7188. Ask for extension 54059 (St Thomas’ Hospital) or extension 53499 (Guy’s Hospital).

Ref number: 3993/VER2
Currently under review

A list of sources is available on request

Contact us

To change or cancel your appointment

Phone: 020 7188 8887 if you need to change or cancel your appointment.

Monday to Friday, 9am to 5pm.

If you urgently need to contact the team on a Saturday or Sunday

Call reception desk: 020 7188 7188 and ask for extension 54046

For questions or concerns

Please contact one of the pre-assessment nurses on 020 7188 7188 and ask for extension 54052.

If you have any other questions about the procedure, contact the endoscopy unit staff for advice.

The nurse in charge: 020 7188 7188 and ask for extension 54059

Do you have any comments or concerns about your care?

Contact our Patient Advice and Liaison Service (PALS)

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