The Phases Of A Clinical Trial

For a new drug or other medical treatment regimen, the clinical trial is its first tested use in humans. Clinical trials are designed to minimize the risks posed to participants. The dose of the treatment and the number of volunteers start out low to monitor for any adverse health effects. Once a useful dose range is established, the number of volunteers slowly increases to study efficacy of the treatment, while continuing to screen for side effects and toxicity.

To formalize this careful progression and monitoring, clinical trials are divided into phases. Each clinical trial phase has its own set of goals and precautions.

Phase 0 Trial

Some clinical trials for new drugs include a phase 0. In this phase, a small number of human participants (10 to 12, on average) receive a low dose of the drug. Since the dose and number of subjects are so low, a phase 0 trial requires far less pre-trial data than a phase 1 trial does.

Phase 1 Trial

The primary goal of a phase 1 trial is to establish an appropriate dose range for the treatment, while monitoring for any adverse side effects. This clinical trial phase usually requires around 20 to 80 volunteers.

Phase 2 Trial

In a phase 2 trial, the number of volunteers increases to as many as several hundred. Researchers monitor patient response to see if the treatment works as intended, and determine how the treatment affects the human body. The safety of the treatment is also closely watched.

Phase 3 Trial

In a phase 3 trial, the number of human subjects is once again increased, this time to around 1,000 to 3,000 volunteers. In a phase 3 trial, researchers continue to test a treatment’s effectiveness and may compare it to existing treatment regimens. The increased number of volunteers also allows researchers to collect substantial data on the frequency and severity of side effects.

Phase 4 Trial

Phase 4 is typically conducted after the treatment has been approved for use. In this phase, researchers gather data on how the treatment is working long-term, and may use that data to fine-tune the treatment recommendations given to patients.


Clinical Trials. (2007). Understanding clinical trials. Retrieved November 30, 2010, from

National Cancer Institute. (2010). Cancer clinical trials. Retrieved November 30, 2010, from

National Cancer Institute. (2007). New approaches to cancer drug development. Retrieved November 30, 2010, from