What are the phases of drug development?

Drug development refers to the phases when a drug gets studied in humans via clinical trails. Typically (but not always) there are 3 phases to drug development before an investigational agent can be considered for regulatory approval, with each phase including a larger number of patients in clinical trials.

Phase I trials are used to get a preliminary assessment of a drug’s safety as well as identification of a dose that is safe. It will often test several different doses to get an idea of how increasing the dose impacts adverse events (side effects) in patients. Phase I trials are typically in the low 10s of patients.

If a drug seems safe in Phase I and continues to remain a potential opportunity, it may be tested in a larger Phase II trial. Here, additional data on safety can be gathered while also getting some insight into potential efficacy. Phase II trials can help derisk further development by informing a go/no-go decision for Phase III, which is the costliest phase of drug development.

Phase III trials typically include anywhere from 100s to several 1000s of patients and will typically test the drug against current standard of care. They include registrational trials which are designed in accordance with what FDA may be looking for in order to grant approval for a specific indication.

Phase IV trials may further study a drug after it has been granted approval by the FDA including for additional insights in various populations or over a longer time-course of treatment.