How Long Does Each Clinical Trial Phase Last?

The duration of a clinical trial depends on several factors, including how long it takes to enroll participants and the main questions that the trial is designed to answer. In general, trial duration also varies by trial phase. Phase I trials are the shortest. Most Phase I trials take less than a year to complete. If a Phase I trial is studying the effects of a single dose of a drug, it may take only few months from start to finish. Phase II and Phase III trials are longer because these trials begin to study the longer-term effects of an experimental treatment. Phase II trials often last up to 2 years, while Phase III trials may last up to 4 or 5 years. Phase IV trials may last for several years while researchers assess the effects of a drug after it has been approved. A participant’s time in a trial is often shorter than the full trial duration. Participants may complete a clinical trial in, for example, 2 years, but the entire trial might take 4 years to complete. This is because participants join the trial at different times.