Clinical trials phases show how far along a trial is in the research process. The Food and Drug Administration (FDA) requires new treatments to go through 3 phases of clinical trials to be approved for use. The phases are:
Phase 1 – These trials test new treatments to see if they are safe and to find out the best way to give them to patients. These are small trials, usually with fewer than 30 people. These treatments have been tested on animals. Often, patients in Phase 1 trials are the very first people to try them. When Phase 1 trials show good results, they will usually move to Phase 2.
Phase 2 – These trials test how well the new treatment will work to treat a disease. Sometimes there might be multiple treatments in the same trial and patients will be randomly chosen to get one or the other. This is important to make sure treatments are compared fairly. When Phase 2 trials show good results, they will usually move to Phase 3.
Phase 3 – These trials test to see if the new treatment is better than current treatment that has already been approved by the FDA. Patients who join Phase 3 trials may be randomly chosen to have either the new treatment or the current treatment. Sometimes the patient and the doctor won't know which treatment is given. This makes sure treatments are compared fairly. When Phase 3 trials show good results, they are often approved by the FDA and available to treat all patients without being part of a clinical trial.
Watch this video
from the National Cancer Institute, which explains clinical trial phases.