A clinical trial is a scientifically constructed investigation of a treatment (e.g. medication) that involves patient volunteers. The volunteers usually take a medicine or some other form of therapy, to allow researchers to test how effective the therapy is, and whether or not it is safe. Often there are two groups of patients, one that takes the new treatment and one that does not.
All drugs must be tested in order to prove their safety and efficacy before they can be marketed to the public. Clinical trials can take up to 10 years to complete.