Phase I

Phase I studies are the first step in testing a medical intervention on people and are primarily concerned with assessing the safety. They usually involve a small number of healthy volunteers (10-100) all of whom are given the same treatment.

The design of Phase I studies enables the initial assessment of how the human body reacts to the medical intervention. For example how a drug is absorbed, metabolised, and excreted. During this Phase the side effects at different dosages will also be examined.

Typically a Phase I study takes several months to complete.

Phase II

Once a medical intervention has been proved to be safe, Phase II trials begin to test effectiveness and aim to establish whether the experimental drug is of potential benefit to people.

This Phase may last from several months to two years and typically involves between 100 and 300 people. The effectiveness of the medical invention is assessed by comparing it with the standard treatment. In some cases, such as where no effective treatment is available for the condition, the new medical intervention is compared to a placebo. (See "What is a placebo?).

Often, Phase II studies are "blinded" or "double blinded", (see "What is a blinded or double blinded study"). This allows comparative information regarding the relative safety and effectiveness of the experimental medical intervention to be gathered.

Phase III

Phase III clinical trials are undertaken where a medical intervention has shown promise in the Phase I and II. The aim is to gain a definitive understanding of the effectiveness, benefits and potential side effects of the medical intervention under examination.

The trial may also make comparisons to standard treatments available, and nowadays may make assessments of participant's perceptions of their quality of life during the trial.

Typically a Phase III trial will last two to three years and involve large groups of people (300-3,000), often from a number of countries. Having such a large number of participants assists researchers in detecting small but clinically meaningful differences between experimental medical treatments.

Within Phase III trials, not all patients receive the new treatment or medical intervention being researched as these studies are usually randomised. Randomisation is a research method that assists in preventing biased results or bias. The participants must agree to be randomly assigned (ie assigned by chance) to one group or another. One group will be given the medical intervention that is being tested, whilst the other will receive the comparison treatment ("control" treatment; see "What is a placebo" and "What is a control or control group").

Once the Phase III trial is completed, researchers may request approval for marketing from the appropriate government authorities.

Phase IV

Phase IV trials are initiated after a medical intervention has been approved by government authorities and is available on the market. There are several objectives of Phase IV trials:

• to compare a medical intervention with others already available on the market


• to assess long term safety


• to monitor impact on different populations


• to monitor the medical interventions long term effectiveness and impact on the patients quality of life


• to determine the cost-effectiveness of a drug therapy relative to other traditional and new therapies