Clinical trials can sometimes be described in research phases, which are defined by the Food and Drug Administration (FDA). For further information, including how these research phases are designed, see the FDA’s website on clinical research.
Early Phase 1: This phase describes exploratory trials that are conducted before Phase 1 to investigate how a drug affects the body. There is limited human exposure in this phase, and there are no therapeutic or diagnostic goals.
Phase 1: This phase is used to focus on the safety of a drug with a small cohort of healthy human subjects, numbering between 20 and 100. The goal of the Phase 1 clinical trial is to determine any adverse events and how often they occur. Phase 1 trials typically last several months.
Phase 2: This phase’s main goal is to collect data on how effectively a drug works in people with specific conditions and/or diseases, and continues to look for side effects. In this phase, subjects who are given the drug might be compared to another group of participants who receive a different treatment or placebo. Phase 2 trials typically recruit several hundred participants with the condition/disease, and can take up to 2 years to complete.
Phase 3: This research phase gathers more information on drug safety and its efficacy. This is achieved by studying different populations, dosages, and other drug combinations. Adverse effects are monitored. These studies can recruit between 300 and 3,000 volunteers and last between 1 and 4 years.
Phase 4: This phase occurs after the FDA has approved the drug for marketing. Phase 4 studies include post-market safety monitoring, and collect more data on drug safety and efficiency. This research process also has several thousand volunteers who have the disease or condition.