Participating in Clinical Trials

Frequently Asked Questions

13. What are the phases of clinical trials?

Clinical trials of drugs are usually described based on their phase. The U.S. Food and Drug Administration typically requires Phase 1, 2 and 3 trials to be conducted to determine if the drug can be approved for use.

A Phase I trial tests an experimental product on a small group of often healthy people (20 to 80), to judge its safety and side effects, and to find the correct drug dosage.

A Phase II trial uses more people (100 to 300). While the emphasis in Phase 1 is on safety, the emphasis in Phase 2 is on effectiveness. This phase aims to obtain preliminary data on whether the drug works in people who have a certain disease or condition. This phase can last several years.

A Phase III trial is a large study using several hundred or more participants (1,000 to 3,000). This phase compares the experimental drug or procedure to a placebo or standard treatment, to make sure it is safe and works well. If the U.S. Food and Drug Administration agrees that the trial results are positive, they will approve the experimental drug or device.

A Phase IV trial for drugs or devices takes place after the U.S. Food and Drug Administration approves their use. A device or drug's effectiveness and safety are monitored in large, diverse populations. Sometimes the side effects of a drug may not become clear until more people have taken it over a longer period of time.