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Diet advice and bowel preparation for your colonoscopy


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Having a colonoscopy can feel daunting, and it’s natural to have questions about the procedure and how to prepare. This information will walk you through what you’ll need to do in the days before your appointment. Please take the time to read this carefully at least 4 days before your appointment. It’s particularly important if you are taking medication as they may need to be stopped. If you have any further questions or concerns, please do not hesitate to speak to a doctor or nurse caring for you.

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What is a colonoscopy?

This is where a soft, flexible tube with a camera at the end is passed through your back passage (anus) and up inside your bowel by a specially trained doctor or nurse. You should have received the leaflet Having a colonoscopy and this explains the procedure in more detail. 

Why do I need to prepare for this procedure?

Your bowel needs to be empty for the colonoscopy, so that the doctor or specialist nurse can see the lining of your bowel. To achieve this you will need to change your diet and take specific preparations for a few days before the procedure. 

If you are taking medication 

Please tell the nursing staff as soon as possible if you:

  • are taking any medicines to thin the blood or to prevent blood from clotting (anticoagulants or antiplatelets, such as warfarin, rivaroxaban or clopidogrel). You do not need to call if you only take aspirin
  • have diabetes and are on insulin
  • have any allergies to any medicines
  • you are allergic to latex.

Please let us know if you are taking any regular medicines at your pre-assessment appointment. This includes anything you buy yourself over the counter, or any herbal or homeopathic medicines. 

How can I prepare for a colonoscopy?

This advice guides you on how to prepare in the days leading up to your colonoscopy. You need to follow the actions on the days mentioned. Please note that your appointment can be on any working day, but if your appointment is on a Tuesday, 4 days before your colonoscopy would be the previous Friday. 

Please make sure you follow these instructions carefully. If you do not, your bowel may not be clear and your examination may have to be repeated.


  • 4 days before your colonoscopy


    • Eat and drink as normal.
    • Stop taking any medicines that contain iron* as the active ingredient (these will have iron, ferrous, ferric or feredetate in the name of the medicine).
    • Continue to take all other medicines as usual.
  • 3 days before your colonoscopy


    • Eat and drink as normal.
    • Stop taking any medicines, including over the counter medicine that contain codeine phosphate*.
    • Stop taking any medicines that contain loperamide*. 
    • Continue to take all other medicines as usual.

    *A list of what your medicines contain should be featured on the leaflet contained in the box the medicine came in. If you are unsure about whether your medicines contain iron, codeine phosphate or loperamide, please ask your pharmacist, doctor or nurse caring for you. Or call the Pharmacy Medicines Helpline on 020 7188 8748 (open 9am to 5pm, Monday to Friday).

  • 2 days before your colonoscopy


    • Drink plenty of fluids, aim for two litres (about 8 to 10 glasses) per day. Please see guide below.
    • Start restricted eating having small meals from the guide below as this will help the bowel preparation medicine work properly.
    • Do not eat foods containing fibre.


    You can eat or drink

    Well cooked, lean beef, lamb, ham, veal or pork

    Fish, shellfish or poultry

    White rice, noodles or white pasta

    Cereal including cornflakes and crisped rice cereal, but not bran

    White bread or toast

    Rich Tea or other plain biscuits 

    Eggs, especially boiled or poached


    Milk, cream, cottage cheese

    Potatoes without the skins; boiled, creamed, mashed or baked

    Soya and tofu

    Gravy made from stock cubes (plain or cornflour can be used to thicken)

    Sugar or sweetener

    Clear jelly

    Water, cordial, fizzy drinks, clear strained soup or clear stock


    Do not eat or drink

    Fruit, including fresh, dried and canned

    Brown rice, brown pasta

    Hi-fibre breakfast cereals (including porridge oats)

    Pulses or lentils

    Hi-fibre, multi-seeded, 50/50 or wholemeal bread



    Roasted, deep fried (chips)




    Red jelly

    Red cordial, or fruit juice

  • The day before your colonoscopy


    • Take your regular morning medicines unless you have been instructed otherwise.
    • Have a light breakfast, such as boiled egg on toast (white bread), no later than 9am.
    • After breakfast do not eat any solid food until after your examination. 
    • Drink clear fluids only today. This may include water, squash, fizzy drinks, clear strained soup or clear stock. Aim for two litres (about eight to ten glasses) in total throughout the day not including the bowel preparation.
    • You can drink tea or coffee with small splash of milk.   
    • It is best to avoid alcoholic drinks.
    • Stop taking any bulk-forming laxatives now. These include: Isphaghula (such as Fybogel®), Methylcellulose (as in Celevac®), Bran and Sterculia (as in Normacol®)

    Try to take your usual morning medicines before 10am, and your evening medicines after 9pm. Medicines taken between these times may not be fully absorbed because of the bowel preparation solutions you will be taking. If you need any advice on when to take your medicines, please contact the Pharmacy Medicines Helpline on 020 7188 8748 (open 9am to 5pm, Monday to Friday). 

  • On the day of your colonoscopy


    • Stop taking any laxatives and continue to avoid any medicines containing iron, loperamide, or codeine.
    • You should continue to take all of your medications as normal, unless you have been told otherwise by the doctor or endoscopy nurse.
    • Drink plenty of clear fluid – you can drink up to 3 hours before your examination. 
    • Leave valuables at home – do not bring them with you to the Endoscopy Department.
    • You may want to bring a dressing gown, slippers and something to read.


Bowel preparation solutions

We will provide you with bowel preparation solutions which are strong laxative sachets. You will have either our standard preparation called CitraFleet® or Picolax® (sodium picosulphate), Klean-Prep® or Moviprep®. You may be given senna tablets as well. 

If the doctors are concerned about your kidneys or if you have kidney disease they will prescribe you Klean-Prep® or Moviprep®. These preparations are less likely to affect your kidney function than our standard preparation. You may be required to have a blood test before and after the procedure to check your kidney function. Please let us know if:

  • You are unable to drink a large volume of fluid.
  • You are restricted on the volume of fluid you are allowed to drink daily.
  • You have been told by a doctor that your kidneys are not working properly. 

How should I take the bowel preparation?

Please follow the instructions and time schedules in this information and not the instructions on the packet. 

The instructions are different depending on which bowel preparation your doctor has prescribed you – you can find the specific set of instructions for your prescribed solution as follows. The times that you need to take your medicine will also depend on whether you are having your colonoscopy in the morning or afternoon.

The solutions tastes better if served chilled, so you may wish to mix in advance and keep in the fridge. Use within 24 hours of preparation.


  • If you are prescribed Sodium Picosulphate (Picolax® or CitraFleet®) sachets and senna tablets


    On the day before the procedure

    4pm: Take four senna tablets from the pack provided with a glass of water.

    5pm: Mix one sachet of sodium picosulphate with approximately 150ml of water. Do this in a large glass or jug, as the mixture may fizz over. Stir the solution for two to three minutes. If the solution becomes warm, allow it to cool before you drink it. You may add more water if you wish to dilute it. Please drink the preparation at once; or if drinking slowly finish within an hour.

    On the morning of the procedure

    6am for a morning appointment or 10am for an afternoon appointment: Take the second sachet of sodium picosulphate, preparing it in the same way as before.

  • If you are prescribed Klean-Prep®


    On the day before the procedure

    2pm: Empty the contents of one sachet of Klean-Prep® into one litre of water and stir until dissolved. Drink this over the course of an hour / an hour and a half.

    4pm: Dissolve the second sachet of Klean-Prep® and drink as above. 

    6pm: Dissolve the third sachet of Klean-Prep® and drink as above. 

    On the morning of the procedure

    6am for a morning appointment or 10am for an afternoon appointment: Dissolve the fourth sachet of Klean-Prep® and drink as above. 

  • If you are prescribed Moviprep®


    On the day before the procedure

    4pm: Empty one sachet A and one sachet B of Moviprep into 1 litre of water and stir until dissolved. Drink this over the course of 1 hour to 1.5 hours.

    On the morning of the procedure

    6am for a morning appointment or 10am for an afternoon appointment: Dissolve the second sachets of A and B Moviprep and drink over 1 hour to 1.5 hours. 

  • If you are prescribed senna tablets


    Senna is a natural effective laxative for the relief of occasional constipation. For safe use:

    • Keep out of reach and sight of children.
    • Store in the original container.
    • Check the foil is not broken before use. If it is, do not take that tablet.
    • Do not take if you know you are allergic to any of the ingredients.

    Possible side effects

    You may experience temporary mild stomach pains. If this gets serious or if you experience other symptoms after taking this product, tell your doctor or pharmacist.

    Consult your doctor or pharmacist if: 

    • you have sharp or persistent stomach pain
    • your abdomen is tender to the touch or hurts when you move
    • you accidentally take too much senna
    • you have been told by your doctor that you have an intolerance to some sugars. 
    • Do not store above 25ºC. Do not freeze. Do not use after the expiry date.

    Do not store above 25ºC. Do not freeze. Do not use after the expiry date.


Will I have to stay near a toilet once I have started taking the bowel preparation?

Yes, stay near a toilet once you have started taking the bowel preparation solution. You should expect frequent bowel movements and diarrhoea, starting within 3 hours of the first dose and continuing until after your last dose. Please note: The timings of the bowel preparation medication are designed so that the effects wear off before you are due to leave home and travel to the hospital. 

Will I have any side effects?

You may have some bloating or stomach cramps – this is normal. You can use a barrier cream, such as zinc and castor oil on your bottom to prevent soreness during your frequent trips to the toilet. 

Remember to keep drinking clear fluids to stop you from becoming dehydrated. As a guide, try to drink about one glass every hour. Signs that you are dehydrated include dizziness, headache and confusion.

Please report any allergic reactions to the preparation such as rash, itchiness, or redness to the endoscopy unit or your GP. In very rare cases you may have a reaction that causes swelling of the face, lips, tongue or throat. If this happens seek medical help as soon as possible.

If you vomit at any stage after taking the bowel preparation solution, or if you have any concerns regarding this preparation, please telephone the endoscopy unit on 020 7188 3221 (St Thomas’ Hospital) or 020 7188 1728 (Guy’s Hospital). 

If you are taking the contraceptive pill 

If you are taking the contraceptive pill it may not be effective once you have taken the bowel preparation, so you must use other precautions (such as condoms) to avoid pregnancy. Please speak to your GP, pharmacist or family planning clinic for further advice on contraception.

If your health has deteriorated

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

If your health has deteriorated since seeing the doctor or specialist nurse in 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). 

If you need to change or cancel your appointment

Please call 020 7188 8887 if you need to change or cancel your appointment.

If you have questions or concerns

For questions about preparing for your colonoscopy, please contact one of the pre-assessment nurses on 020 7188 7188 and ask for extension 54052 (Mon to Fri, 9am to 5pm).

If you have any other questions about the procedure, contact the endoscopy unit staff for advice (Monday to Saturday, 9am to 5pm):

For the nurse in charge: 020 7188 7188 and ask for extension 54059 

For St Thomas' reception desk: 020 7188 7188 and ask for extension 54046

For Guy's Hospital: 020 7188 1728

In case of emergency: Out of hours (6pm until 8am the next day, and on Saturdays and Sundays) call 020 7188 7188 and ask to be put through to the on-call gastroenterology registrar.

Please see our Gastroenterology and Hepatology webpage regarding other endoscopy frequently asked questions and answers. 



Ref number: 3993/VER2

Date published: Feb 2019 | Review date: Feb 2022

© 2020 Guy’s and St Thomas’ NHS Foundation Trust

A list of sources is available on request


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