Clinical trials that evaluate new treatment approaches are done in phases. Each phase has a different purpose, helping researchers to answer different questions. If a new drug or treatment approach does not seem promising in an early-phase trial, the research can be stopped. Every clinical trial, regardless of its phase, has a “Principal Investigator” (often a medical doctor), who is responsible for leading the trial.

All clinical trials, including those focused on cancer, follow these established phases:

Phase I. In phase I trials, researchers study the safety of a new drug (or drug combination) or a new dose of a currently-approved drug. Researchers try to find the lowest dose that will still be effective, determine which patients are able to tolerate the drug and study side effects that might occur. These trials usually include a small number of participants and take place in research centers, where the participants can be closely monitored.

Phase II. The purpose of this phase is to determine if the treatment is effective and to learn more about its safety. In some phase II trials, participants are “randomized,” meaning they are selected by chance to either receive the treatment being studied, or one that is already being used to treat their type of cancer. Phase II trials typically involve a few hundred participants and, like phase I trials, usually take place in research centers. (It is important to note that in cancer clinical trials, participants are generally not given a placebo, which is a pill or liquid that contains no active ingredient. A placebo is only used when there is no standard treatment against which a new treatment can be compared.)

Phase III. In this phase, the treatment being tested is given to hundreds or even thousands of participants, who are often randomized as in phase II trials. There are a number of purposes of this phase:

  • To confirm how well the treatment works against the cancer
  • To learn about side effects that might not have been seen during earlier phases
  • To compare the new treatment approach with current approaches
  • To collect information that will allow the new treatment approach to be used safely

Because of their size, phase III studies are often carried out in multiple sites, including by doctors in private practice, in community hospitals and in designated cancer centers.

Phase IV. In phase IV trials, researchers study treatments after they have been approved by the FDA. These trials are designed to learn more about the treatment’s risks and benefits and the best way for it to be used. Phase IV trials help doctors understand how safe and useful the treatment will be over the long term.