Clinical trial phases describe the sequential stages of testing new drugs, biologics, or medical devices in humans. Each phase builds on the previous one to assess safety, efficacy, dosage, and potential side effects before a treatment is approved for general use.
Phase- I: Testing a drug on a very small number of healthy volunteers to establish overall safety and identify side effects, and determine the safe & tolerable dose levels.
Phase- II: Testing a drug on a small number of people who have the condition the drug is designed to treat.
Phase- III: Testing a drug on a large number of people who have the condition the drug is designed to treat.
Phase- IV: Investigating uses of the drug for other conditions, on a broader patient base (e.g. elderly patients), or for longer term use.
Example
A pharmaceutical company develops a new biologic drug for rheumatoid arthritis. After laboratory and animal testing, they begin a Phase I clinical trial with 40 healthy volunteers to establish safety and dosage. Once safety is confirmed, they proceed to a Phase II trial with 150 patients who have rheumatoid arthritis to test effectiveness and refine dosing. This is followed by a Phase III trial involving thousands of patients worldwide to confirm efficacy and monitor side effects before seeking regulatory approval. After approval, Phase IV studies track the drug’s performance in the general population and evaluate potential new indications.