What is a colonoscopy? Why it is done and what to expect

What a colonoscopy is

The US National Cancer Institute (NCI) describes colonoscopy as a procedure to look inside the rectum and colon for polyps, abnormal areas or cancer. A colonoscope - a thin, tube-like instrument with a light and a lens - is passed in through the rectum. It may also carry a tool to remove polyps or take tissue samples, which are then checked under a microscope (NCI: Colorectal Cancer Screening (PDQ)). The NHS describes it as a test to check inside your bowels using a thin tube with a small camera inside it (NHS: Colonoscopy).

Polyps are growths on the inner lining of the bowel. NCI explains that most colorectal polyps are adenomas, which may develop into cancer, and that removing polyps larger than 1 centimetre (pea-sized) may lower the risk of colorectal cancer (NCI: Colorectal Cancer Prevention (PDQ)).

Why a colonoscopy is done

The NHS says you might be referred for a colonoscopy if:

• you are bleeding from your bottom or there is blood in your poo;
• you have diarrhoea or constipation that does not go away;
• you are losing weight or feeling very tired for no reason;
• your bowel is being checked for polyps or for conditions such as Crohn's disease, diverticular disease or bowel cancer.

The NHS adds that most of the time a colonoscopy will not find anything to worry about. Symptoms like these always deserve a doctor's assessment - see our guide to warning signs to see a doctor about.

Colonoscopy and bowel cancer screening

Colonoscopy is one of several ways to screen for colorectal (bowel) cancer. NCI lists stool tests that look for hidden blood (including the faecal immunochemical test, FIT), stool DNA tests, sigmoidoscopy (which looks at the lower part of the bowel only), colonoscopy, and virtual colonoscopy (a CT scan of the bowel).

• United States: the US Preventive Services Task Force (USPSTF) recommends screening for colorectal cancer in adults aged 45 to 75, and selective screening from 76 to 85 depending on health, past screening and preferences. Several test strategies are acceptable, colonoscopy among them. This applies to people at average risk without symptoms (USPSTF: Colorectal Cancer Screening, 2021).
• England: NHS bowel cancer screening uses a FIT home test kit, sent by post every 2 years to people aged 50 to 74. If blood is found, you are usually offered a colonoscopy. The NHS notes that blood in a sample does not always mean cancer (NHS: Bowel cancer screening).

People with a higher risk - for example a past bowel polyp, inflammatory bowel disease, or a family history of certain inherited conditions such as Lynch syndrome - are outside these average-risk recommendations and may need a different plan. Ask your doctor, and see is cancer hereditary? For how screening fits together, read cancer screening explained.

Getting ready: bowel preparation

For the specialist to see the bowel lining clearly, the bowel needs to be empty. The NHS explains that once you are referred you will be given laxatives, or told where to get them, along with information about:

• when and where the procedure will be;
• what you can eat and drink in the days before;
• when to stop eating and drinking;
• how to take the laxatives (this is called bowel preparation, or "prep").

The NHS says you may get diarrhoea a few hours after the first dose, so plan to stay at home near a toilet. It also asks you to tell the hospital if you are pregnant, to bring details of the medicines you take, and to follow any instructions about your medicines. The NHS warns that if the laxatives are not taken as advised, the specialist may not be able to do the colonoscopy. Instructions differ between hospitals and products, so this page deliberately does not give prep doses or diets - use the ones you were given, and call the unit if anything is unclear.

What happens during the procedure

The NHS describes the day as follows. You stop eating on the day, and are told when to stop drinking. A healthcare professional explains what will happen and you change into a gown. The test may be uncomfortable but should not hurt; depending on the hospital you may be offered painkillers, sedation (medicine through a small tube in the arm to help you relax) or gas and air.

• The thin, flexible tube is passed into your bottom and through the large bowel.
• A small amount of gas or water is pumped in to open up the bowel, which can make you feel bloated or as if you need the toilet, and you may have cramps.
• Any polyps may be removed, or a small tissue sample (biopsy) taken - you will not feel this.

The NHS says the colonoscopy itself takes 30 to 45 minutes, but you may be at the hospital for around 2 hours in total.

Afterwards

You will usually be told whether any polyps were removed or samples taken, and then rest in a recovery area. The NHS says you cannot drive for 24 hours if you had sedation, so someone will need to take you home. Bloating or stomach cramps for 2 to 3 hours, and a little blood in your poo or from your bottom for a couple of days, are common.

Possible risks

The NHS calls colonoscopy a common and straightforward procedure with rare complications, which can include a reaction to the sedation, bleeding, and damage to the bowel wall (perforation). NCI notes that tears and bleeding are serious when they happen and occur more often when a biopsy or polyp removal is done. Ask your specialist about your own risks before the test.

Getting your results

The NHS says you should get a letter or call with your results a few weeks after the test. Results may show:

• Normal - no polyps or signs of cancer. People at higher risk may still be asked to have colonoscopies every few years.
• Polyps - removed and tested, with advice about any treatment or a future check-up colonoscopy.
• Another bowel condition - such as piles, Crohn's disease, diverticular disease or ulcerative colitis.
• Bowel cancer - you would see a cancer specialist as soon as possible. The NHS notes that the earlier bowel cancer is found, the more likely treatment is to be effective. Our guides to how cancer is diagnosed and questions to ask your doctor may help with next steps.

Sources

• National Cancer Institute: Colorectal Cancer Screening (PDQ) - Patient Version
• National Cancer Institute: Colorectal Cancer Prevention (PDQ) - Patient Version
• US Preventive Services Task Force: Colorectal Cancer Screening (2021)
• NHS: Colonoscopy
• NHS: Bowel cancer screening

Published and last reviewed: 21 October 2026.