Clinical trials are structured into four phases:
In Phase I clinical trials, researchers test a study drug in a small group of people (20-80) for the first time to evaluate its safety, determine a safe dosage range, and identify side effects.
In Phase II clinical trials, the study drug is tested in a larger group of people (100 to 300) to measure its effectiveness and further evaluate its safety.
In Phase III clinical trials, the study drug is tested in large groups of people (1,000 to 3,000) to confirm its effectiveness, monitor side effects, compare it to approved standard treatments, and collect information that will allow the study drug to be used safely.
In Phase IV clinical trials, the drug is tested after it has been marketed to collect information about its effect in various populations and about any side effects associated with long-term use.