Clinical trials are conducted in phases from phase 1 through phase 4. Phase 1 aims to test safety and usually involve a small number of people. Phase 2 aims to evaluate effectiveness, and usually involve a larger number of people. Phase 3 aims to compare two or more treatments or treatment strategies and monitor side effects. Results from these trials allow drugs to treatment recommendations being made and drugs licensed. Phase 4 trials are post-marketing studies, and collect further information on use of treatments in clinical practice.

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