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The Phases of a Clinical Trial

A clinical trial can usually be categorized as falling into one of four stages or "phases."

Phase 1

This is the first experiment using an investigational new drug in humans. Usually, healthy participants are enrolled in Phase 1 studies; however, participants with the disease being studied may under certain circumstances be enrolled. Phase 1 studies are designed to determine how the drug is broken down in the human body and how it interacts with the human body. Phase 1 studies reveal some of the side effects associated with increasing doses. When possible, some Phase 1 studies provide early evidence of effectiveness. Researchers use information from Phase 1 studies to design Phase 2 studies. The total number of participants included in Phase 1 studies varies with the drug, but is generally in the range of 20-80.

Phase 2

These are the first effectiveness studies of a drug in humans. Taking the drug at the doses and on the schedule found to be safe in Phase 1 trials, researchers administer the drug to participants with the disease or condition of interest. During Phase 2, researchers collect additional safety and effectiveness data, study short-term side effects and risks, and collect additional information about the proper dose and dosing schedule. Phase 2 studies typically involve control groups, are closely monitored, and are conducted in a relatively small number of participants, usually not more than several hundred participants.

Phase 3

These studies are expanded, longer-term research studies, performed after Phase 1 and Phase 2 studies have shown some evidence of effectiveness. Phase 3 studies are intended to gather the additional information about effectiveness and safety that is needed to evaluate the overall benefit/risk relationship of the drug. Researchers collect additional information about drug-related bad side effects, including the less common side effects. Several hundred to several thousand participants may be enrolled. At the conclusion of a properly designed Phase 3 trial, the new drug will be found to be inferior, equivalent, or superior to the standard treatment.

Phase 4

The intent of these "post-marketing surveillance" studies is to learn more about the drug after it has been approved by the U.S. Food and Drug Administration. In Phase 4 studies, researchers gather additional information about an approved drug's risks, benefits, and best uses associated with large-scale usage in "real-life conditions." These studies can also include trials of different doses or schedules of administration, other stages of disease, cost studies, quality-of-life studies, or use of the drug over a longer period of time.