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Phases of a Clinical Trial

The U.S. Food and Drug Administration (FDA) requires at least three phases before a new medication can be approved.

Phase I trial

In a Phase I trial, an experimental drug, device, or treatment is tested on a small group of people, usually with 20 to 80 participants. The purpose of a Phase I trial is to determine the safety of the treatment, device, or drug. This phase may be called nonblinded (or "open-label"), because both researchers and volunteers know what medication is being used.

Phase II trial

In a Phase II trial, further study of a new drug, device, or treatment with 100 to 300 participants takes place with participants who have the condition or disease for the target of the treatment. A Phase II study of an investigational treatment is designed to determine if the drug or device is an effective treatment for a specific disease, has any side effects, or has any risks.

Phase III trial

In a Phase III trial, large groups of people (1,000 to 3,000) participate to allow researchers to collect data on safety and efficacy of a drug or treatment and to compare the new treatment to the standard treatment or to placebo (an inactive treatment used to compare the effects of the experimental treatment with no treatment). This phase may be randomized and blinded. In a randomized study, participants in the trial are assigned randomly (like flipping a coin, for example) to the groups with the new treatment, standard treatment, or placebo, when relevant. A blinded study means patients and/or doctors do not know which treatment they are receiving (single-blinded and/or double-blinded).

Phase IV trial

In a Phase IV trial, further study takes place to determine the risks, benefits, and best use of the drug or treatment once the treatment is being marketed. Usually thousands of people are involved in this phase.

To learn more about clinical trials in general, visit

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