Most clinical research involves testing how a new drug progresses in an orderly series of steps called phases. This process allows researchers to ask and answer questions in a way that results in reliable information about the drug, but also protects patients.

Research studies are typically classified into one of three phases:

• Phase I trials: These first studies evaluate how a new drug should be given (by mouth or injected into the blood or muscle), how often it should be administered and at what dosage. A Phase I trial usually enrolls only a small number of healthy volunteers, sometimes as few as a dozen.
• Phase II trials: A Phase II trial continues to test the safety of the drug in people with the medical condition to be treated and begins to evaluate how well the new drug works, and determines the appropriate dosage.
• Phase III trials: These studies test a new drug, a new combination of drugs or a new surgical procedure in comparison to the current standard in people with the medical condition to be treated. A participant is typically assigned to the standard group or the new group at random (called randomization). Phase III trials often enroll large numbers of people and may be conducted at many doctors' offices, clinics and centers nationwide or even worldwide.