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What are the Phases of Clinical Trials?

Most discoveries of new treatments are the result of laboratory experiments. When researchers are ready to evaluate a new treatment in people, they carefully design clinical trials to test the treatment’s effectiveness, its safety, and its side effects. Clinical trials follow an orderly progression of steps (or phases) to take the medication or other type of treatment from discovery to approval by the Food and Drug Administration.


Phase 1

Phase 1 includes the initial introduction of an investigational new drug into humans. Phase 1 studies are typically closely monitored and may be conducted in patients or normal volunteer subjects. These studies are designed to determine the metabolism and pharmacologic actions of the drug in humans, the side effects associated with increasing doses, if possible, to gain early evidence on effectiveness. During Phase 1, sufficient information about the drug’s pharmacokinetics and pharmacological effects should be obtained to permit the design of well-controlled, scientifically valid, Phase 2 studies. The total number of subjects and patients included in Phase 1 studies varies with the drug, but in generally in the range of 20 – 80.


Phase 2

Phase 2 includes the controlled clinical studies conducted to evaluate the effectiveness of the drug for a particular indication or indications in patients with the disease or condition under study and to determine the common short-term side effects and risks associated with the drug. Phase 2 studies are typically well controlled, closely monitored, and conducted in a relatively small number of patients, usually involving no more than a several hundred subjects.


Phase 3

Phase 3 studies are expanded controlled and uncontrolled trials. They are performed after preliminary evidence suggesting effectiveness of the drug has been obtained, and are intended to gather the additional information about effectiveness and safety that is needed to evaluate the overall benefit-risk relationship of the drug and to provide an adequate basis to physician labeling. Phase 3 studies usually include from several hundred to several thousand subjects.


Phase 4

At times, even after FDA approval, additional information is needed on the effectiveness or the safety of a new treatment. If so, a phase IV clinical trial is conducted to provide such information.


Other Trials

Other supportive care, prevention, and screening trials are not done in phases, but instead compare groups of people using another strategy (counseling, behavior changes, detection method) with those who do not.


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