All medical interventions have the potential to cause harm. This is particularly important in the case of cancer screening because the intervention is offered to people who are, or at least believe themselves to be, in good health, and the tolerance limit of harm must accordingly be low.

Screening may cause harm in several ways.1 If the screening test is not highly sensitive, false negative results may induce reassurance and create a “certificate of health effect.” In other words, people who have received a false negative test result may ignore symptoms or continue to engage in risky behaviour. Then the test itself can cause harm—for example, colonoscopy as a consequence of colorectal screening may lead to colonic perforation or other complications, and this must be monitored. Given that most people who are screened will not have disease, unnecessary psychological morbidity may also be created. Finally, screening inevitably leads to a degree of overdiagnosis —that is, people will be found to have disease that …