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.