Phase I studies are used to learn the “maximum tolerated dose” of a drug that does not produce unacceptable side effects. Patient volunteers are followed primarily for side effects, and not for how the drug affects their disease. The first few volunteer subjects receive low doses of the trial drug to see how the drug is tolerated and to learn how it acts in the body. The next group of volunteer subjects receives larger amounts. Phase I studies typically offer little or no benefit to the volunteer subjects.
Phase II studies involve a drug whose dose and side effects are well known. Many more volunteer subjects are tested, to define side effects, learn how it is used in the body, and learn how it helps the condition under study. Some volunteer subjects may benefit from a phase II study.
Phase III studies compare the new drug against a commonly used drug. Some volunteer subjects will be given the new drug and some the commonly used drug. The trial is designed to find where the new drug fits in managing a particular condition.
Phase IV studies (sometimes known as “post-marketing” or “surveillance” studies) gathers additional information about an approved drug’s side effects. In some cases the FDA requires a Phase IV study be conducted after drug approval.