Participating in Clinical Trials
Frequently Asked Questions
13. What are the phases of a clinical trial?
A clinical trial usually includes three phases. In some cases, four phases may be required.
A Phase I trial tests an experimental treatment 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 is similar to a Phase I trial but uses more people (100 to 300) to find out if the experimental treatment is effective and safe. 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. Some side effects that didn't show up in Phase II may show up in a Phase III trial because many more people are tested.
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.