Clinical trials are conducted in four phases as defined by the FDA. These are:
Phase I studies are usually conducted with typically 20-80 healthy volunteers to determine the drug’s most frequent side effects and how the drug is metabolized and excreted. The emphasis in Phase I is on safety.
If Phase I studies don’t reveal unacceptable toxicity, the clinical trial moves to Phase II.
Phase II studies aim to obtain preliminary data on whether the drug works in people who have a certain disease or condition. Typically a few dozen to about 300 subjects are enrolled in this phase. For controlled trials, patients receiving the drug are compared with similar patients receiving a different treatment–usually an inactive substance (placebo), or a different drug. The emphasis in Phase 2 is on effectiveness. But safety continues to be evaluated, and short-term side effects are studied.
Phase III studies begin if evidence of effectiveness is shown in Phase II.
Phase III studies gather more information about safety and effectiveness, studying different populations and different dosages and using the drug in combination with other drugs. Typically anywhere from several hundred to about 3,000 subjects are involved in this phase.
If approved after Phase III, the new drug is approved for marketing in the United States by the FDA.
Phase IV studies begins once a pharmaceutical or medical treatment has been approved by the FDA for marketing to the public. This expanded study explores the effectiveness of a drug or treatment protocol over an extended period of time for any side effects with its long-term use.