What are the phases of a clinical trial?

Clinical trials consist of four phases to test a treatment:¹

• Phase 1: Focuses on the safety of a drug or other intervention. This phase usually involves 20 to 80 healthy participants and is used to determine the most frequent and serious adverse events, to find the correct dosing, and discern how a drug is broken down and excreted by the body.^{2, 3}
• Phase 2: This is where preliminary data are studied to determine a drug’s effectiveness in people with certain conditions or diseases. Here, safety and short-term adverse events continue to be evaluated. ⁴
• Phase 3: More information is being gathered in this stage regarding a drug’s safety and efficacy by studying different populations and dosages and how it interacts with other drugs.⁵ These studies typically involve several hundred to about 3,000 people.⁶ Once the FDA determines that the results from the trial are positive, it will approve the experimental drug or intervention.
• Phase 4: This stage of the trial occurs after the FDA has granted the drug approval for marketing.⁷ During this phase there is ongoing monitoring to determine the safety and effectiveness of the drug or intervention in larger, more diverse populations.⁸