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Colonoscopy preparation: what to eat, how to take the bowel prep and what to expect
In my experience, most people cope with the colonoscopy better than they expect, while the preparation beforehand causes much more uncertainty, usually about what they can eat, when the laxative will start working, whether they will be up all night and what to do with their regular medicines.
Good bowel preparation gives a clear view of the lining of the colon, which is important because small or flat polyps can be missed when stool remains and occasionally the examination then has to be repeated.
The instructions supplied by your endoscopy unit always take priority over a general guide. Different bowel preparations have different timings, and morning and afternoon appointments are usually prepared for differently.
What should I eat before a colonoscopy?
Most units advise a low-fibre diet for two or three days before the examination. Fibre is healthy in ordinary circumstances, but seeds, skins and coarse food are slow to clear from the bowel.
Foods that are usually suitable include:
- White bread, white rice and plain pasta
- Eggs, fish and skinless chicken
- Potatoes without the skin
- Smooth yoghurt without fruit pieces
- Well-cooked vegetables without skins, seeds or stalks
Foods usually avoided include:
- Wholemeal bread, brown rice and high-fibre cereal
- Salad, raw vegetables and sweetcorn
- Beans and lentils
- Nuts, seeds and popcorn
- Berries, dried fruit, fruit skins and pips
The exact point at which solid food must stop will be in your instructions. Some units allow a light meal on the day before the procedure; others move to clear fluids earlier.
What counts as a clear fluid?
Clear fluids usually include water, black tea or coffee without milk, clear apple juice, pale squash, clear fizzy drinks and strained broth. Avoid smoothies, drinks containing milk, juice with pulp and anything coloured red or purple.
Do not rely on water alone throughout the preparation. Clear drinks containing some sugar and salt can help with energy and hydration, provided they are suitable for you. People with diabetes need individual advice.
Keep drinking within the times stated in your instructions. The final cut-off depends on the timing of the procedure and whether sedation is planned.
Taking the bowel preparation
Most modern regimens divide the laxative into two doses. The second dose is taken closer to thecolonoscopy because this usually gives a cleaner bowel. Follow the stated times even if the bowel already appears empty after the first dose.
You should expect frequent, watery bowel movements, often beginning within one or two hours, although it can take longer, so stay close to a toilet and keep the evening and morning free.
A few practical measures make the process easier:
- Chill the solution if the instructions allow
- Drink it steadily rather than trying to finish it quickly
- Use a straw if you dislike the taste
- Apply barrier cream before bowel movements become frequent
- Keep a choice of permitted clear drinks available
- Contact the clinic if repeated vomiting prevents you completing the preparation
How do I know whether the preparation has worked?
Towards the end, what you pass should be watery, pale yellow or almost clear, without solid material. A small amount of yellow colour is normal.
If it remains brown, cloudy or contains solid stool after the full preparation, contact the endoscopy unit before travelling. Do not take extra laxative unless the unit advises it.
Medicines need to be discussed before the day
Tell the clinic about every prescribed medicine, injection, supplement and over-the-counter treatment when thecolonoscopy is arranged, rather than waiting until the day of the test.
Blood thinners
Warfarin, apixaban, rivaroxaban, edoxaban, dabigatran and antiplatelet medicines such as clopidogrel need an individual plan. The advice depends on why the medicine was prescribed and whether a polyp may need to be removed. Do not stop a blood thinner without instructions.
Diabetes treatment
Fasting and bowel preparation can affect glucose levels. Insulin, sulfonylureas and SGLT2 inhibitors may need particular changes. The plan will depend on the medicine, the appointment time and your usual glucose control. Bring your diabetes medication and glucose-monitoring equipment with you.
Weight-loss and GLP-1 medicines
Semaglutide, tirzepatide, liraglutide and similar medicines can slow stomach emptying. Current advice depends on the drug, your symptoms, the form of sedation and local anaesthetic policy. Tell the unit when booking and follow the instructions they give you.
Iron, constipation treatments and anti-diarrhoeal medicines
Iron tablets, fibre supplements and medicines such as loperamide are often stopped several days beforehand because they can interfere with bowel cleansing. The timing varies, so use the instructions from your unit.
Also tell the team if you have kidney disease, heart failure, a stoma, severe constipation or previous difficulty completing bowel preparation. These can affect which preparation is safest and whether extra measures are needed.
What happens on the day?
The endoscopist will check your medical history, the quality of the preparation and your consent before starting. Thecolonoscopy itself commonly takes about 30 to 45 minutes, although removal of polyps can add time.
Options for comfort may include intravenous sedation, pain relief or gas and air, depending on the unit and your health. If intravenous sedation is used, you will need someone to collect you. You must follow the unit’s restrictions on driving, alcohol, machinery and important decisions afterwards.
The air used during the examination can cause temporary wind, bloating or mild discomfort, which usually settles, whereas severe or increasing abdominal pain, heavy or persistent bleeding, fever, breathlessness or feeling faint requires urgent medical advice.
When should colonoscopy be considered?
Colonoscopymay be recommended for rectal bleeding, iron-deficiency anaemia, a persistent change in bowel habit, inflammatory bowel disease assessment, surveillance after previous polyps, a significant family history or an abnormal bowel-screening test.
The right investigation depends on the symptom and the results already available, so a consultation can be useful when it is unclear whethercolonoscopy, another test or treatment should come first. In my practice, previous results and the reason for requesting the examination are reviewed before a procedure is arranged.
Arranging a private colonoscopy in Milton Keynes
Harmony welcomes self-referrals as well as insured patients. A consultation allows time to review symptoms, medicines, previous investigations and the reason for consideringcolonoscopy. If the procedure is appropriate, the preparation and the arrangements can then be planned properly.
Frequently asked questions
Is the bowel preparation worse than the colonoscopy?
Many patients find the preparation more inconvenient than the examination. Knowing the timings, having suitable drinks ready and staying close to a toilet makes it more manageable.
Can I drink coffee?
Black coffee is usually allowed during the clear-fluid period. Milk, cream and non-dairy creamer are not clear fluids. Follow the list supplied by your unit.
What if I vomit the preparation?
Pause briefly and try again more slowly. Contact the endoscopy unit if vomiting continues or you cannot keep the fluid down. An incomplete preparation may mean the examination cannot be completed properly.
Should I take my usual tablets on the morning?
Many medicines can be taken with a small sip of water, but blood thinners, diabetes treatment, iron tablets and some weight-loss medicines need specific advice. Check before the day of the procedure.
How long will I be at the hospital?
The examination commonly takes 30 to 45 minutes, but admission, consent and recovery add time. Sedation usually means a longer recovery and the need for an escort home.
Written and clinically reviewedby Dr Arjun Prakash, Consultant Gastroenterologist and Advanced Interventional Endoscopist
Dr Arjun Prakash provides private gastroenterology consultations in Milton Keynes, including assessment for bowel cancer risk, screening colonoscopy, family history, previous polyps and concerning bowel symptoms.
