What is a phase in a clinical trial?
Most clinical trials are carried out in steps called phases. Each phase is designed to find different information. Patients may be eligible for studies in different phases depending on their general condition, the type and stage of their condition, and what therapy, if any, they have already had. Patients are seen regularly to determine the effect of the treatment, and treatment is always stopped if side effects become too severe. (See the phase descriptions below) Phase I studies are primarily concerned with assessing the drug’s safety. This initial phase of testing in humans is done in a small number of healthy volunteers (20 to 100), who are usually paid for participating in the study. The study is designed to determine what happens to the drug in the human body–how it is absorbed, metabolized, and excreted. A phase I study will investigate side effects that occur as dosage levels are increased. This initial phase of testing typically takes several months. About 70 percent of exper
