Colonoscopy: how long it takes, the prep and when colon cancer screening starts

What is a colonoscopy, and how is colon cancer screening done?

A colonoscopy is usually done with sedation, so for many women the test itself is the quiet part of the day. A GI doctor (gastroenterologist) passes the camera through the rectum and looks along the colon for polyps, small growths that are common and usually harmless. Colonoscopy polyps can be removed there and then and sent to a lab, so looking and treating often happen in one visit (NIDDK, 2026).

Colonoscopy prep is the part that takes the most planning, and it happens at home. The aim is an empty bowel, so the camera has a clear view. The NIDDK describes a clear-liquid diet for a day or more beforehand, then laxative doses that are usually taken the night before and the morning of the procedure. The NHS notes that diarrhea (spelled diarrhoea in NHS leaflets) tends to begin a few hours after the first dose, so many women keep that evening free and stay close to a bathroom. Preparations for a colonoscopy vary a little between clinics, and the sheet your own unit gives you is the one to follow.

Afterward, you rest until the sedation wears off, and the NHS says you are usually told then whether anything was removed or biopsied.

The other main route is the FIT, a stool test completed at home and sent to a lab, which looks for blood too small to see. In the US, either test is a recognized starting point (USPSTF, 2021), while the NHS bowel screening program in England begins with the home test kit.

Does colonoscopy screening work, and why start at 45?

The reassuring part of the evidence is how simple the idea is. Most colon cancers start as polyps (NIDDK, 2026), and polyps can be found and removed early. Screening is a way of looking while any changes are still small and before symptoms appear.

The US start age moved from 50 to 45 in the US Preventive Services Task Force's 2021 recommendation. The reason is a gradual shift in who is affected: the American Cancer Society (2026) reports that colorectal cancer in people under 50 rose about 2.9% per year from 2013 to 2022. That is a trend across a whole population, and it does not predict any one woman's risk. It means an earlier first look is now on offer, and the American Cancer Society guideline sets the same starting age.

Because bowel cancer risk rises with age, routine screening carries on to 75, and from 76 to 85 it becomes an individual decision shaped by your health and earlier results (USPSTF, 2021). In the UK, the NHS home kit continues every 2 years to 74.

A home stool test and a colonoscopy work together. After an abnormal stool test or other non-colonoscopy test, the American Cancer Society guideline calls for a follow-up colonoscopy to look for the cause, and in the UK a positive NHS home kit usually leads to one as well. A positive kit is a reason for a closer look, not a diagnosis.

How often is colon cancer screening needed, and what are the risks?

For average-risk adults in the US, a screening colonoscopy is repeated every 10 years (USPSTF, 2021). The other USPSTF options are a FIT every year, a stool DNA-FIT every 1 to 3 years, CT colonography every 5 years, or flexible sigmoidoscopy every 5 years. England uses one route instead: a home FIT kit every 2 years for ages 50 to 74 (NHS, 2026).

Serious problems from a colonoscopy are uncommon. The NIDDK (2026) puts perforation, a small tear in the bowel wall, at about 3 in 10,000 screening colonoscopies and bleeding at about 15 in 10,000. Bleeding can appear up to 2 weeks later and is more likely after a polyp has been removed. The NHS describes perforation as a rare risk too.

Screening choices often fit naturally into a well woman exam, where family history and the other checks due this year can be talked through together.

Screening is designed for people without symptoms (USPSTF, 2021). Bleeding or a changed bowel habit is worth a doctor's visit at any age rather than a wait for the next screening date, and the same goes for the red flags listed on this page, such as unexplained weight loss or symptoms that are getting worse.

What do the studies show, and who might need a different plan?

The studies behind these guidelines describe adults at average risk, so the ages and intervals above are a starting point and some women are offered a different plan. The American Cancer Society (2026) lists groups who may begin before 45: those with a family history, a personal history of polyps, inflammatory bowel disease, Lynch syndrome, familial adenomatous polyposis or earlier radiation to the pelvis.

Timing for these groups is set individually, with a GI doctor in the US or through a GP referral in the UK, around your own history.

The guidelines also leave the choice of test open: the USPSTF sets out several, each with its own interval.

Next steps

A good place to start is a short conversation with your usual doctor about which test appeals to you, a home stool test (FIT) or a colonoscopy, and whether your family history points to a different timetable. In the UK, the first step is usually the home kit when it is offered. If anything on the red-flag list sounds familiar, it's worth booking a visit now rather than waiting for the next screening date. Screening is not one size fits all, so the test and the timing are yours to decide.