Clinical trials explained

Clinical trials have four phases:

Phase I


Phase I studies are for first-time experimental treatments. Participants may include healthy volunteers or patients who have a specific disease or a range of diseases or conditions. Most Phase I clinical trials are single-arm studies, which means only one group of people are receiving the experimental treatment. Only a small number of people (10-80) receive the new treatment in this phase.

A Phase I clinical trial seeks answers to the following questions:

  • Is the experimental treatment safe?
  • How much of the experimental drug is safe to give a person?
  • What are the side effects?

Phase II


Phase II clinical trials involve more patients (about 100-200). The experimental treatment is studied for a longer time. Phase II clinical trials help determine if the new treatment works well for patients with a certain disease or for a group of similar diseases or conditions. Information from these studies will help doctors and drug companies decide if more studies should be done and which patients might benefit from this new experimental treatment.

Phase III


Phase III clinical trials can include thousands of patients. The purpose of this clinical trial phase is to see if the experimental treatment works for patients with a specific disease and to monitor for side effects. In a Phase III trial, the new experimental treatment is compared to other known standard treatments. Usually, these studies involve more than one group of people receiving the experimental treatment.

Phase IV


Phase IV clinical trials occur after the U.S. Food and Drug Administration (FDA) has approved a drug or device for doctors to use or prescribe.

These trials help researchers learn:

  • More about the side effects and safety of the drug or device.
  • What the long-term risks and benefits are.
  • How well the drug or device works when used in a much larger group of people.