Phase 1 clinical trials are done to see if an experimental medication or treatment is safe. After a treatment is tested in the lab or on animals, it enters a phase 1 clinical trial that is done with humans. These trials usually involve only a small number of people to determine if a drug or treatment is safe, and to determine the best dose of a drug and how it should be given (whether orally or intravenously).
If a treatment appears safe at the end of a phase 1 clinical trial, it may then enter a phase 2 trial, a study done to see if a treatment is effective. If a drug or treatment is deemed safe in a phase 1 trial and effective in a phase 2 trial, it will then enter a phase 3 clinical trial. Phase 3 clinical trials are much larger, and are done to see if a treatment is not only safe and effective, but works better or has fewer side effects than treatments currently available.