New medicines undergo considerable testing before being released for use in the community. This article describes the testing process, and highlights some of its limitations.

Only a small proportion of potential new medicines make it through the entire testing process: the rest are withdrawn at some stage.

Animal studies

Potential new medicines are first tested on animals to determine whether they appear to be safe enough and effective enough to justify further testing on humans.

Clinical trials

Clinical trials (testing on people) are then carried out in four stages. The researchers conducting the trials must adhere to strict guidelines that specify how they must conduct the trials, and ensure that participants fully understand what the trial involves and provide their consent.

Phase 1 trials
Potential medicines are first tested on 20-80 healthy young people (usually men) to begin determining how the human body reacts to the medicine, and to ascertain the dose that can safely be given to people in the next phase of testing.

Phase 2 trials
The potential medicine is then given to 50-100 people who have the disease the medicine is intended to treat. The aim of this testing is to further assess the medicine's effects on the body. The information is used to ascertain the smallest dose that is effective in most people, and the largest dose that most people can tolerate without experiencing unacceptable side effects.

Phase 3 trials
The next stage of testing is designed to assess the medicine's safety and effectiveness, and to determine the most appropriate dose.

To achieve this, 1000-3000 people with the disease the medicine is intended to treat are randomly divided into two groups. One group is given the potential medicine. The other group is given a placebo (dummy) pill or another medicine that has been used to treat people with the disease for some years. This allows the effectiveness of the new medicine to be compared with no treatment or a known treatment.

People taking part in Phase 1-3 trials are monitored closely for side effects, so a picture of the medicine's side effects can start to emerge. However, the picture is always incomplete. In particular, it is unlikely to include the uncommon and rare side effects. Identifying such side effects is not usually possible until many thousands of people have used the medicine. This may occur only when the medicine has been used in the community for several years.

Governments use the results of Phase 3 trials to determine whether the medicine is safe enough and effective enough to be approved for use in their country. Not all medicines obtain government approval.

Only a small percentage of new
medicines make it through
the entire tesing process

Phase 4 trials
All new medicines are monitored after being released for use in the community. Some also undergo further Phase 4 testing. Such testing may follow for several years many thousands of people undergoing treatment with the medicine.

Phase 4 testing allows researchers to learn more about the medicine, and helps them to resolve uncertainties about the medicine's safety, side effects and most appropriate use. For example, it may be used to determine whether the medicine should be the first treatment tried for a condition, or whether it should be used only if other treatments have not worked.

Phase 4 trials take a long time, but they often provide more useful information than the earlier trials because they test the medicine in 'real-life situations'.