A clinical trial is a study of the efficacy and value of treatments or interventions, and most such trials involve comparing the treatment or intervention under investigation to a control group.

This should be a prospective process (as opposed to a retrospective one), in which participants or subjects are identified at the start of the process and followed forward in time. Investigators implement carefully defined interventions. Data are then gathered, evaluated and analysed and the results may be published. Interventions may include treatments, screening programmes, preventative measures and diagnostic tests.

Types of trial

Phased clinical trials

Phase I trials

In phase I trials, new treatments are administered to small groups of people. Often, useful information on safety and efficacy has been obtained from animal studies before the phase I trial. People who participate in phase I trials are often those who have failed to improve on existing, conventional therapies, or they may be healthy volunteers. One of the main purposes of this type of trial is to ascertain what the maximum safe dose of the study drug is and how it is metabolised. This maximum dose is usually referred to as the maximum tolerated dose (MTD).

Phase II trials

After the safety and maximum tolerated dose have been established, the investigators may proceed to a phase II trial. Typically, small groups of patients are started on different doses and frequencies of administration. Neither a phase I or a phase II trial is randomised.