Jefferson Health

Phases of Clinical Trials

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Most clinical research involves the testing of a new drug in an orderly series of steps, called phases. This allows researchers to ask and answer questions in a way that results in reliable information about the risks and benefits of the drug.

  • Phase I Trials. These first studies in people evaluate how a new drug should be given (by mouth, injected into the blood, or injected into the muscle); how often; and what dose is safe. A Phase I trial usually enrolls only a small number of patients.
  • Phase II Trials. A Phase II trial continues to test the safety of the drug, and begins to evaluate how well the new drug works. Phase II studies usually focus on a particular type of cancer. 
  • Phase III Trials. These studies test a new drug, a new combination of drugs, or a new surgical procedure in comparison to the current standard. A participant will usually be assigned to the standard group or the new group at random. Phase III trials often enroll a large numbers of people and may be conducted at many doctor's offices, clinics, and cancer centers nationwide.
  • Phase IV Trials. The purpose of Phase IV trials is to evaluate the side effects, risks, and benefits of a drug over a longer period of time and in a larger number of people than in a Phase III clinical trial.