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Phases of clinical trials

Many clinical trials to develop new interventions are conducted in phases. In the early phases, the new intervention is tested in a small number of participants to assess safety and effectiveness. If the intervention is promising, it may move to later phases of testing where the number of participants is increased to collect more information on effectiveness and possible side effects.

Clinical trials of biomedical interventions typically proceed through four phases.

Phase I Clinical Trial

Phase I clinical trials are done to test a new biomedical intervention for the first time in a small group of people (e.g. 20-80) to evaluate safety (e.g. to determine a safe dosage range and identify side effects).

Phase II Clinical Trial

Phase II clinical trials are done to study an intervention in a larger group of people (several hundred) to determine efficacy (that is, whether it works as intended) and to further evaluate its safety.

Phase III Clinical Trial

Phase III studies are done to study the efficacy of an intervention in large groups of trial participants (from several hundred to several thousand) by comparing the intervention to other standard or experimental interventions (or to non-interventional standard care).  Phase III studies are also used to monitor adverse effects and to collect information that will allow the intervention to be used safely.

Phase IV Clinical Trial

Phase IV studies are done after an intervention has been marketed. These studies are designed to monitor the effectiveness of the approved intervention in the general population and to collect information about any adverse effects associated with widespread use over longer periods of time.  They may also be used to investigate the potential use of the intervention in a different condition, or in combination with other therapies.

Other Clinical Trials

Researchers may also conduct exploratory studies, sometimes referred to as ‘Phase 0 trials’ or ‘pilot studies’.  These come before Phase I trials and are used to test how the body responds to an experimental drug. In these studies, small doses of the new drug are given once or for a short time to a very limited number of people.

Clinical trials of diagnostic tests are sometimes divided into exploratory phases, challenge phases and advanced phases to see how effective and how accurate the tests are.