Harms or risks may result from performing the screening test or from additional tests required to clarify abnormal screening test results, and from treating the lesions detected through screening.

The faecal occult blood test (FOBT) and faecal immunochemical test (FIT) screening tests are painless and safe. However, FOBT or FIT may detect blood in your stool unrelated to bowel cancer or polyps. If blood is found in your stool, you will be offered further tests with colonoscopy (see “Follow-up colonoscopy” below). People sent for further testing due to abnormal screening test results sometimes experience anxiety, but this is usually not long-lasting.

Many patients experience at least some degree of discomfort or pain with flexible sigmoidoscopy, but less than with colonoscopy. Minor complications (nausea, feeling faint or dizzy, or abdominal pain) occur in less than 1 out of every 150 flexible sigmoidoscopies. Major complications, including heavy bleeding or a tear through the wall of the bowel, occur in about 1 out of 3,000 flexible sigmoidoscopies. Severe pain immediately after the examination occurs in about 1 out of 50 patients.

As with screening using the guaiac FOBT or FIT, an abnormal screening examination requires further testing with colonoscopy (see “Follow-up colonoscopy” below).

Many patients experience at least some degree of discomfort or pain with colonoscopy. The laxative bowel preparation (i.e. the preparation required before attending colonoscopy) is generally perceived to be the worst part of the procedure. Screening colonoscopy avoids the need for another examination in many cases in which polyps or small cancers are detected because it also covers the upper half of the bowel. Major complications have been reported in the European Union in about 1 to 5 out of 1,000 examinations. Complications are more likely in colonoscopies in which polyps are found and removed. In extremely rare cases, colonoscopy may result in death.

Follow-up colonoscopy is a colonoscopy performed to follow-up an abnormal guaiac FOBT or FIT result or to check the upper half of the bowel in people with abnormal sigmoidoscopy screening results. It carries somewhat higher risks of complications than screening colonoscopy, due to the higher chance of polyps or cancers being discovered and removed. Many of the polyps removed during screening would not have progressed to cancer during a person's lifetime, but it is not possible to predict which polyps will develop into cancer. Major complications have been reported in Europe in about 1 out of 200 follow-up colonoscopies. There is a greater risk of complications from surgery if the polyp or cancer cannot be removed by colonoscopy. In extremely rare cases (in 1 out of 10,000 examinations) colonoscopy may result in death, but the number of bowel cancer deaths prevented by screening is much greater.