In Phase I a small number of healthy volunteers are exposed to the research treatment. The method of delivery as well as the dosing regimen is explored during this phase, and side effects are noted. Before Phase I studies begin, experiments comparing the new treatment with the drug of choice for the planned condition have been done in laboratory models and in animal studies, as well as extensive animal toxicity studies.

In Phase II, the effectiveness of the new treatment is characterized. The new drug is examined in patients using strict design criteria - appropriate monitoring, use of adequate controls, careful exploration of the effective and safe dose range, etc.

In Phase III, large studies are done to compare the new medicament against a recognized standard treatment. Again, the studies must be well-controlled and well-conducted, to provide clear cut evidence of safety and effectiveness for the new drug regulatory authorities (e.g. FDA).

Clinical trials do not end after the drug is approved; in what is called Phase IV, the new drug is constantly monitored for safety, and it's safety and effectiveness are studied in special populations of patients - children, the very-old, people with liver disease, etcetera.

Click here to see a graphical presentation of the drug discovery process designed by the FDA.