Clinical trial
In Trial design & structure
Also known as: clinical study, interventional study
A clinical trial is a research study in which people are assigned to an intervention according to a plan, in order to evaluate its effects on health outcomes. The intervention can be a medicine, a device, a procedure, or a change in behavior or care. It differs from an observational study, where researchers watch what happens without assigning treatment.
| Phase | The main question it answers |
|---|---|
| Phase I | Is it safe and tolerable in people, and how does the body absorb and clear it? Usually the first time a treatment is given to people. |
| Phase II | Is there early evidence that it works, and at what dose, while safety is still being studied? |
| Phase III | Does it work and how safe is it, compared with a placebo or the current standard? This is the core evidence for approval. |
| Phase IV | After approval: what happens over the long term, in everyday practice, and in groups not well represented earlier? |
Where the rule comes from
The international standard for running a clinical trial is ICH E6(R3), Good Clinical Practice, adopted on 6 January 2025. Its 11 principles (Section II) apply to every trial in its scope, and Annex 1 sets out the duties of the ethics committee, the investigator and the sponsor. E6(R3) does not define the phases; they follow common regulatory usage.
Example
Randomly assigning patients to a new blood-pressure drug or to placebo is a clinical trial. Following a group of patients who chose the drug with their own doctor is an observational study.
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