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What Are the Different Phases (Stages) of Clinical Trials?

Updated December 17, 2014

New treatments are tested in clinical trials after several years of research in the laboratory and with animals. The federal Food and Drug Administration (FDA) must agree that the treatment is safe before human testing can begin.

Human testing occurs in four phases, or steps—each is a separate trial. Data from each phase must be approved by the FDA before the next phase can begin.

  • Phase I: First human test step. Measures safety; best dosage and delivery method (by mouth, injected); side effects. Enrolls small number of participants.
  • Phase II: Determines effectiveness; watches for side effects. Usually not randomized (see definition below). Usually has 25 to 100 participants.
  • Phase III: Compares new treatment to current standard; measures side effects. Often randomized (participants randomly assigned to treatment groups) and double-blinded (researcher and participant do not know which therapy participant is receiving). When phase III is completed, new treatment may receive FDA approval. Can include hundreds to thousands of participants.
  • Phase IV: After FDA approval. Monitors effects of long-term use and compares new medicines to others. Many times, there is no phase IV trial. Thousands of people are involved in a phase IV trial.

Read more about clinical trials and the providers who helped us write this page in our Guide to Understanding Breast Cancer Treatment Research Studies.