A colonoscopy is a camera test of the large bowel. For most people the hardest part isn’t the test itself but the bowel preparation the day before. This guide explains why you might have one, how to make the prep easier, your options for comfort during the test, what happens if polyps are found, and what the results mean.
Key points
- A colonoscopy uses a thin, flexible camera passed through the bottom to look at the lining of the large bowel (colon) and rectum.
- Your bowel has to be completely empty, so you’ll follow a low-fibre diet and drink a bowel-cleansing preparation beforehand.
- You can usually choose sedation, gas and air (Entonox), or both, to stay comfortable.
- Small growths (polyps) can be removed during the test, which helps prevent bowel cancer.
- Serious complications, such as bleeding or a tear in the bowel, happen in about 1 in 1,000 tests.
Why you might have a colonoscopy
- a change in bowel habit, or bleeding from the bottom
- iron deficiency anaemia[5]
- a positive FIT (poo) test, either from your GP or from the NHS Bowel Cancer Screening Programme, which now sends kits to everyone aged 50 to 74 in England[2,3]
- ongoing diarrhoea, especially if a test called faecal calprotectin suggests inflammation. See our guide to IBS.
- to check on inflammatory bowel disease, or because polyps were found before[4]
If your symptoms are in the upper gut, you may have a gastroscopy on the same day.
Preparing for your colonoscopy
A clean bowel means a clearer view and a shorter, more accurate test. Your hospital will send instructions and a bowel preparation kit. Follow their timings exactly, as they vary between hospitals and preparations.[1]
Typical preparation
- About a week before: stop iron tablets. Check with the unit about blood thinners and diabetes medicines.[1]
- A few days before: avoid nuts, seeds, sweetcorn and fruit and vegetable skins, and switch to a low-fibre diet (white bread, white rice, pasta, chicken, fish, eggs). Your instructions will say how many days.[1]
- The day before: clear fluids only, and start drinking the bowel preparation at the times given. You’ll need to stay close to a toilet.
Tips that make bowel prep easier
- Chill the preparation in the fridge and drink it through a straw.
- Drink each glass steadily over 15 to 20 minutes rather than sipping for ages.
- Suck a slice of lemon or a boiled sweet (not red) between glasses.
- Keep drinking extra clear fluids to stay hydrated.
- Use a barrier cream or soft wet wipes to stop your bottom getting sore.
- Your poo should end up as clear or pale yellow liquid. If it isn’t, call the unit for advice.
Taking other tablets? Bowel preparation can stop other medicines being absorbed, including the contraceptive pill. Check your instructions, and use extra contraception for the rest of that cycle if advised.
Bowel prep matters more than people think
I’m currently training in colonoscopy, and I led an audit across several hospitals comparing two common bowel preparations. I wrote the protocol, collected and analysed the data, and presented it at a regional conference.
That work showed me how much the quality of the preparation shapes the test. A clean bowel lets us see the lining properly, makes small polyps easier to spot, and means you’re less likely to need the test repeated.
So my advice is simple: follow the timings on your instructions, finish the whole dose, and keep drinking clear fluids. If you’re struggling to finish it, ring the unit before giving up, because there’s often a way to help.
Ben JohnsonAdvanced Clinical Practitioner, Gastroenterology
Comfort during the test: your options
| Sedation | Entonox (gas and air) | |
|---|---|---|
| How it’s given | Medicine through a small tube in your arm | You breathe it through a mouthpiece, as and when you need it |
| How you’ll feel | Relaxed and drowsy; you may remember little | Awake; it takes the edge off cramping |
| Going home | You must be collected, and someone should stay with you | Usually you can drive after about 30 minutes |
| Next 24 hours | No driving, alcohol, machinery or signing important documents | No restrictions once it’s worn off |
Some units offer both, or a choice.[1] You can talk it through with the nurse on the day.
During the test
- You lie on your left side. The colonoscope is gently passed into your bottom and around the bowel.
- Air or water is used to open the bowel so the lining can be seen. This can cause cramping and bloating, and you may pass wind. That’s normal.
- You may be asked to change position, or a nurse may press gently on your tummy, to help the scope round the bends.
- Biopsies and polyp removal are usually painless.
- The test usually takes 20 to 30 minutes, sometimes longer.[1]
After the test
- Bloating and wind usually settle within a few hours.
- Your bowel habit may be a little different for a few days.
- If you had sedation, rest for the rest of the day and follow the 24-hour rules above.
Contact the endoscopy unit, NHS 111 or your GP if, after your test, you have:
- severe or worsening tummy pain
- a high temperature
- passing more than a small amount of blood from your bottom
If you are passing a lot of blood, feel faint or the pain is severe, call 999 or go to A&E and tell them you’ve had a colonoscopy.
Risks
- Bleeding or a tear (perforation) in the bowel: about 1 in 1,000. The risk is a little higher if polyps are removed. Very occasionally a blood transfusion or surgery is needed.[1]
- A small number of abnormalities can be missed, which is why good bowel prep matters.[1]
- A reaction to the sedation.
Results
You’ll usually be told what was seen before you leave, and given a written report. Biopsy and polyp results take a few weeks, and are sent to you, your GP or discussed at a follow-up appointment. Results are prioritised if there’s any concern about cancer.[1]
If polyps were found
Polyps are small growths on the bowel lining. They’re very common, especially after 50, and most are harmless. Some types could slowly turn into cancer over many years, so they’re usually removed during the test and checked under a microscope.[4]
Whether you need another colonoscopy depends on how many polyps were found, their size and type. UK guidance recommends a repeat colonoscopy after 3 years for people with higher-risk findings. Most people with only one or two small polyps don’t need a repeat test, and return to routine bowel screening instead.[4]
Frequently asked questions
Is a colonoscopy painful?
It can be uncomfortable, mainly from cramping as air is used and the scope goes round bends. Sedation or gas and air helps most people. Tell the team if you’re uncomfortable: they can adjust things.
Can I eat normally after?
Yes, once you feel ready. Start with something light.
What if they find a polyp?
Most polyps are harmless, but some could become cancer over many years, so they’re usually removed during the test. This is one of the ways colonoscopy helps prevent bowel cancer.
Is there an alternative to colonoscopy?
A CT scan of the bowel (CT colonography) is sometimes used, but it can’t remove polyps or take biopsies, so you may still need a colonoscopy afterwards. Your team will advise which is best for you.
Can I work the next day?
Yes, usually. If you had sedation, take the rest of the test day off.
References
- Harrogate and District NHS Foundation Trust. Having a colonoscopy (patient information, 2023). https://www.hdft.nhs.uk/wp-content/uploads/2015/12/Having-a-Colonoscopy-2023-002.pdf
- NHS England. NHS rolls out lifesaving home testing for bowel cancer to over 50s. January 2025. https://www.england.nhs.uk/2025/01/nhs-rolls-out-lifesaving-home-testing-for-bowel-cancer-to-over-50s
- NICE. Quantitative faecal immunochemical testing to guide colorectal cancer pathway referral in primary care (DG56). 2023. https://www.nice.org.uk/guidance/dg56
- Rutter MD, East J, Rees CJ, et al. British Society of Gastroenterology/Association of Coloproctology of Great Britain and Ireland/Public Health England post-polypectomy and post-colorectal cancer resection surveillance guidelines. Gut 2020;69:201–223. https://gut.bmj.com/content/69/2/201
- Snook J, et al. British Society of Gastroenterology guidelines for the management of iron deficiency anaemia in adults. Gut 2021;70:2030–2051. https://gut.bmj.com/content/70/11/2030