Phase I - evaluates the safety of the medication in a small number of healthy people, determines the appropriate dosage, and identifies side effects.

Phase II - focuses on whether the treatment works (efficacy). It involves a few hundred patients who have the relevant medical condition. Usually one group receives the experimental treatment, and another receives a placebo (in-active medication). In randomized studies, participants are assigned to each group by chance.

Phase III - emphasizes definitive statistical results. It involves giving the medication to several hundred to thousands of patients, and the response is compared with a commonly used treatment. If there is no effective existing treatment, the effects of the new treatment are compared with those of a placebo.

Phase IV - occurs after the medication has been approved by the Food and Drug Administration (FDA) and is available to the public. This post-marketing phase provides information about long-term effectiveness and side effects.