Phase I: This is a clinical study done in a small number of people. Participants can be people who have a certain disease or illness, or they can be healthy volunteers. The number one goal of a phase I study is to find out how safe a new drug is. These studies establish a safe dosage and sometime even a dosing schedule or route of administration. Phase 1 studies may be the first time a certain drug has been given to humans.
Phase II: These clinical studies are done in a larger number of people than phase I studies. The number one goal of these studies is to see if a drug is effective and to closely monitor the safety of the participants and the side effects they experience.
Phase III: These are the largest of the clinical studies, and often health care sites all over the world will participate. The number one goal of these trials is to compare the new treatment to the existing treatment(s), which is sometimes no treatment at all.
Phase IV: These studies take place after a treatment has been approved by the U.S. Food and Drug Administration and are used to study long-term safety and effectiveness of the new treatment. The number of participants varies depending on the disease and how many people are typically affected.