clinical trial(redirected from Phase IIb)
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Four phases of clinical trial are distinguished. Phase I trials usually involve fewer than 100 healthy volunteers who are exposed to a new drug or procedure. Such studies seek to establish optimal dosage and route of administration and to detect adverse reactions. Phase II trials generally involve 200-500 volunteers randomly assigned to control and study groups. These are pilot efficacy studies, with emphasis on immunogenicity in the case of vaccines, and on relative efficacy and safety in the case of drugs, procedures, and devices. Phase III trials, often multicenter, involve thousands of volunteers, randomly assigned to control and study groups. The aim is to generate statistically relevant data. Phase IV trials are conducted after a national drug registration authority (in the U.S., the Food and Drug Administration) has approved an agent for distribution or sale. They may explore specific pharmacologic effects, adverse reactions, or long-term effects.