Phases of a Trial

Clinical trials are usually conducted in stages that build on one another, called phases. Phases are based on the trial’s objective, number of patients, and other aspects of the trial. Each phase is designed to answer certain questions. The five phases of clinical trials are known as Early Phase 1 (formerly Phase 0), Phase 1, Phase 2, Phase 3, and Phase 4. For patients, knowing the phase of a clinical trial is helpful because it offers an idea about what is already known about the treatment studied in the trial.

Early Phase 1 (formerly Phase 0) Early Phase 1 trials are exploratory trials conducted prior to Phase 1 trials that involve very limited human exposure to the treatment being studied.

Phase 1 Phase 1 of a clinical trial emphasizes safety. The goal is to see how the body handles and metabolizes the drug in order to determine the ideal dose with the fewest adverse effects. Phase 1 trials are usually conducted on a small number of healthy volunteers. Specific dosages of drugs determined to be safe enough move on to the next phase of the trial.

Phase 2 This phase of research describes a clinical trial that determines whether a drug is effective in patients with a certain condition or disease. Safety continues to be a priority, as does the identification of side effects. Phase 2 trials are performed on a larger group of people than in Phase 1 (100–300) and sometimes introduce the use of an inactive placebo for comparison purposes.

Phase 3 Drugs that are found to work in Phase 2 move on to Phase 3. In a Phase 3 clinical trial, a drug’s efficacy and safety continue to be studied and side effects monitored in larger groups of participants in different populations and with more dosages. Phase 3 trials are sometimes randomized, meaning the different groups in the trial are created according to chance. In these trials the new drug can be used in combination with other drugs or compared to the standard-of-care treatment in order to study its efficacy.

Phase 4 Phase 4 trials occur after a new drug has been approved by the FDA for marketing to the public. These trials involve testing and tracking hundreds or thousands of patients in the general population. Doctors collect more data about the drug’s safety and its efficacy both with and without other treatments to help determine its optimal use.