Phase I Drug Trial
Phase I trials include the initial introduction of an investigational new drug into humans. These studies are typically conducted with healthy volunteers; sometimes, where the drug is intended for use in the patients with a particular disease such patients may participate as subjects. Phase I trials are designed to determine the metabolic and pharmacological actions of the drug in humans, the side effects associated with increasing doses (to establish a safe dose range), and, if possible, to gain early evidence of efficacy. They are typically closely monitored. The ultimate goal of Phase I trials is to obtain sufficient information about the drug's pharmocokinetics and pharmacological effects to permit the design of well-controlled, sufficiently valid Phase II studies. Other examples of Phase I studies include studies of drug metabolism, structure-activity relationships and mechanisms of actions in humans as well as studies in which investigational drugs are used as research tools to explore biological phenomena or disease processes. The total number of subjects involved in Phase I studies is generally in the range of 20-80.

Phase II Drug Trial
Phase II trials include controlled clinical studies conducted to evaluate the drug's effectiveness for a particular indication in patients with the disease or condition under study, and to determine the common short-term side effects and risks associated with the drug. These studies are typically well-controlled, closely monitored, and conducted with a relatively small number of patients, usually involving no more than a few hundred subjects.

Phase III Drug Trial
Phase III trials involve the administration of a new drug to a larger number of patients in different clinical settings to determine its safety, effectiveness, and appropriate dosage. They are performed after preliminary evidence of efficacy has been obtained, and are intended to gather necessary additional information about efficacy and safety for evaluating the overall benefit-risk relationship of the drug, and to provide an adequate basis for physician labeling. In Phase III studies, the drug is used the way it would be administered when marketed. When these studies are completed and the sponsor believes the drug is safe and effective under specific conditions, the sponsor applies to FDA for approval to market the drug. Phase III trials usually include several hundred to several thousand subjects.

Phase IV Drug Trial
Concurrent with marketing approval, FDA may seek agreement from the sponsor to conduct certain post-marketing studies to delineate additional information about the drug's risks, benefits, and optimal use. These studies could include, but are not limited to, studying different doses or schedules of administration than were used in Phase II studies, use of the drug in other patient populations or other stages of the disease, or use of the drug over a longer period of time (21 CFR 312.85).