New medicines are tested in stages, and each phase answers a different question. A plain-language guide to what happens in Phase I, II, III and IV.
Before a new medicine, vaccine or device reaches patients, it is studied step by step. Each phase of clinical research builds on what was learned before and has its own purpose, size and safeguards.
Phase I: Is it safe, and what dose?
Phase I studies are usually the first time a new treatment is given to people. They involve a small group, often healthy volunteers or, for some conditions such as cancer, patients. The main goals are to understand safety, how the body handles the drug and which dose ranges are suitable.
Phase II: Does it work, and at what cost in side effects?
Phase II studies involve a larger group of people who have the condition being studied. They look for early signs that the treatment works and continue to collect safety information to guide the design of larger studies.
Phase III: Is it better than, or as good as, current care?
Phase III studies are larger still, often run across many hospitals and countries. They compare the new treatment with the current standard of care or a placebo, confirm how well it works and identify less common side effects. Their results usually form the basis of a marketing approval application.
Phase IV: What happens in everyday use?
After a treatment is approved, Phase IV or post-marketing studies follow its use in a much wider population over a longer time. They help detect rare or long-term effects and answer practical questions about real-world use.
The same safeguards in every phase
- Approval from the regulator and a registered ethics committee before the study begins.
- Informed consent, explained in a language the participant understands.
- The right to withdraw at any time without affecting regular medical care.
This article is general information and not medical advice. Please speak with your doctor about whether a clinical study may be suitable for you.