Why do clinical trials have eligibility requirements?

Clinical trials have eligibility requirements for a few important reasons. First, they reduce medical differences among participants, so the group being studied is more consistent. Second, they reduce the risk of harm to people who might not tolerate a treatment safely. Third, they limit the trial to people most likely to actually benefit from what's being tested.

These requirements also strengthen the trustworthiness of a trial's results. When participants share certain characteristics, researchers can be more confident that differences seen in outcomes come from the treatment itself, rather than from unrelated differences among the people taking part.

While eligibility requirements can feel restrictive if you don't happen to meet them for a specific trial, they exist to protect participants and to produce results that are meaningful enough to eventually change how cancer is treated for future patients.