How does eligibility work?

Every clinical trial has eligibility criteria: a list of requirements that describe who can and cannot take part. These are usually split into inclusion criteria, which are things you must have, and exclusion criteria, which are things that would rule you out.

Common criteria include age, sex, the type and stage of a condition, previous treatments, other health conditions and current medications. Some trials only recruit people with a specific diagnosis, while others need healthy volunteers with no diagnosis at all.

Eligibility criteria exist for two main reasons. The first is safety: some treatments may not be safe for people with certain health conditions, or may interact with particular medications. The second is science: to answer its research question reliably, a study needs participants whose results can be compared fairly.

Checking eligibility usually happens in stages. An initial pre-screening, often a short questionnaire or phone call, checks the basics. If that looks promising, you attend a screening visit where the research team confirms eligibility with a medical history, examination and any tests the protocol requires.

Not being eligible for a trial is common and is not a judgement about you or how serious your condition is. It simply means that particular study is not the right fit. Criteria differ between studies, so being unsuitable for one trial does not mean you will be unsuitable for others.

If you are not eligible for a study you were interested in, ask the research team whether they know of other trials, and keep searching, because new studies open regularly.