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Clinical trials are conducted in phases. Each phase focuses on answering particular questions about the drug or treatment being studied. In the early phases, the focus is on determining if the treatment is safe. Later phases are used to determine if the treatment is more effective than the standard treatment.

Phase I This phase is usually the first clinical trial that involves humans. The purposes of phase I clinical trials are to determine that the treatment is safe and at what dosage the treatment is most effective treatment with the lowest risk of adverse effects. Phase I clinical trials usually involve 15 to 30 participants who are watched very closely at administration and during the time periods between doses. 

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Phase II After a treatment is shown to be safe, phase II clinical trials are used to determine if the treatment works. Doctors observe the effects of the treatment on the cancer. Does it shrink the tumor? Does it slow or stop disease progression? Does it improve the patient’s quality of life? Does the treatment help the patient live longer than expected with the standard treatment? Phase II clinical trials generally include less than 100 participants.

Phase III The goals for phase III clinical trials include confirm how well the treatment works, compare the new treatment with standard treatments, and collect information on side effects and how to use the treatment safely. A large number of participants (several hundred to several thousand) from all over the country or throughout the world are included in this phase. A process called randomization is used to determine which patients receive the new treatment. A placebo may be used if doctors want to determine if the new treatment will improve the effectiveness of an existing treatment; however, a placebo is not used by itself in cancer clinical trials if a standard therapy is available.

Phase IV This phase is used to gather additional information on the effectiveness of a treatment after it has received approval for use by the US Food and Drug Administration (FDA). Phase IV clinical trials are used to determine the long-term effects of using a treatment. 


American Cancer Society. Clinical Trials: What You Need to Know. Atlanta, GA: American Cancer Society; last medical review/last revised September 2010.

McCabe M, Messner C. Clinical Trials: Improving the Care of People Living With Cancer. New York, NY: CancerCare; 2009.

National Cancer Institute. Taking Part in Cancer Treatment Research Studies. Rockville, MD: US Department of Health and Human Services; revised September 2011. NIH Publication No. 12-6249.