Each phase has a different purpose and helps researchers answer different questions.

  • Phase I trials. Researchers test a drug or treatment in a small group of people (20 to 80) for the first time. The purpose is to study the drug or treatment to learn about safety and identify side effects.
  • Phase II trials. The new drug or treatment is given to a larger group of people (100 to 300) to determine its effectiveness and to further study its safety.
  • Phase III trials. The new drug or treatment is given to large groups of people (1,000 to 3,000) to confirm its effectiveness, monitor side effects, compare it with standard or similar treatments, and collect information that will allow the new drug or treatment to be used safely.
  • Phase IV trials. After a drug is approved by the FDA and made available to the public, researchers track its safety in the general population, seeking more information about a drug or treatment’s benefits, and optimal use.

Reproduced with permission from NIH Clinical Trials and You. NIH does not endorse or recommend any products, services, or information described or offered here by Healthline. Page last reviewed on October 20, 2017.