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Clinical trial phases

Learn about the four clinical trial phases.

Phase I Trials

These first studies in people evaluate how a new treatment should be administered (orally, intravenously, by injection, inhalation, etc.), how often and in what dosage. The primary objective of a Phase I trial is to define the proper dosing and identify side effects. A Phase I trial usually enrolls only a small number of patients.

Phase II Trials

A Phase II trial provides preliminary information about how well the new treatment works and generates more information about safety and benefit. The goal of Phase II studies is to determine the common short-term side effects and risks (if any) associated with a new drug.

Phase III Trials

These trials compare a promising new drug, combination of drugs or procedure with the current standard therapy. Phase III trials typically involve large numbers of patients from doctors’ offices, clinics and centers nationwide. The reason that the Phase III clinical trial has been initiated is that the superiority of one treatment over the other has not yet been firmly established.

If you participate in a Phase III treatment trial, you are likely to be randomized (assigned by chance) to a group receiving either the new intervention or the standard intervention. Neither you nor your physician chooses whether you get the new intervention or the standard treatment.

Trials designed in this way are also called randomized controlled trials. If you are assigned the standard intervention, you receive what experts view as the best treatment available. Experts believe that each treatment is effective but really don’t know which one is better. If you are assigned the new intervention, you receive a treatment that some experts think may have some advantages over the standard.

Phase IV Trials

Some use the term Phase IV to include the continuing evaluation that takes place after FDA approval, when the drug or treatment procedure is already on the market and available for general use. This is also called a post-marketing surveillance study.