What is the final phase of clinical trials that must usually be completed in order for the FDA to approve a new drug?

The FDA reviews all data up to and including Phase 3 clinical trials to approve a drug for use in the United States.

Multiple phases of preclinical and clinical trials are performed to assess a new drug with regards to safety and efficacy.

The discovery or preclinical phase includes studies in the laboratory and studies in animals models to identify potential new drugs. Clinical phases include phase 0 through 4:

Phase 0: First study in human volunteers to understand how the body metabolizes the drug

Phase I: Study in a small number of patients to evaluate safety and dosing

Phase II: Study in larger group of patients to evaluate efficacy and further evaluate safety

Phase III: Study with a large number of patients to confirm efficacy and safety, assess side-effects, and compare the drug to current treatment options or placebo

Phase IV: Post marketing surveillance in many patients to further evaluate safety and long-term safety and efficacy. Performed after approval in real-life patients.

On average it takes more than 10 years from the time a drug enters clinical studies until it is reviewed by the FDA and approximately 1 in 10 drugs entering into clinical trials are approved for use by the FDA.