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Phases of a Clinical Trial

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A clinical trial is done to find out if a medicine or treatment is safe and works well for treating a certain condition or disease.

A medicine or treatment must go through three phases before it is approved for use by the U.S. Food and Drug Administration (FDA). A fourth phase happens after the medicine or treatment has been approved.

  • Phase I: A new medicine is tested for the first time on a small group of healthy people or people with certain conditions or diseases. Researchers check the safety of the medicine or treatment, the best dose or schedule to use, and what types of side effects occur. During this phase, all the people involved in the study (patients, doctors, and researchers) know what medicine is being used. These are called nonrandomized, nonblinded studies.
  • Phase II: The medicine or treatment is tested on a larger group of people with certain conditions or diseases. This phase helps researchers find out how well a medicine or treatment will work to treat a particular problem. Phase II trials are also usually nonrandomized, nonblinded studies.
  • Phase III: The medicine or treatment is tested on even larger groups. The medicine is studied to find out how well it works compared with standard treatment or placebo. Researchers also study whether the medicine improves specific areas in your life, such as how well you are able to keep your usual routine. Most medicines that reach this phase will be considered for FDA approval. During phase III trials, participants receive the study medicine, a placebo, or the standard treatment. Neither the participants, the doctors, nor the researchers know which person is getting which medicine. These are called randomized and double-blinded studies.
  • Phase IV: Medicines are also studied after they are approved. These studies can find new uses for the medicine, different ways to give it, or more safety information. For example, a medicine may be studied to see how well it works for a certain population, such as adults over the age of 65 or a certain racial group.

New combinations of approved medicines can be studied in phase II, phase III, or phase IV trials.

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Last Updated: November 14, 2014
This information is not intended to replace the advice of a doctor. Healthwise disclaims any liability for the decisions you make based on this information.
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