Bookmarks Share Print

Clinical trial process

Clinical trials related to drugs are classified into four phases. The trials at each phase have a different purpose and help scientists answer different questions:

Phase I trials test an experimental drug, vaccine or device in a small group of people to evaluate safety and possible side effects, and to determine how the drug should be used or delivered.

Phase II trials involve more people than Phase I and they are designed to assess the safety and efficacy of an experimental drug, vaccine or device. This phase can last several years.

Phase III trials are usually large studies with many participants. This phase compares the experimental drug or vaccine to a placebo or standard treatment, to evaluate safety and efficacy. Some side effects that were not identified in Phase II may be identified in a Phase III trial because many more people are evaluated. The regulatory health authority, such as the U.S. Food and Drug Administration, will consider the results of clinical trials up to and including Phase III trials when determining whether to approve a new drug or vaccine.

Phase IV trials take place after a regulatory health authority, such as the U.S. Food and Drug Administration, has approved the use of the drug or vaccine. A drug's effectiveness and safety are monitored in large, diverse populations.