Exploring if and how a new drug may work.

These are exploratory studies that often use only a few small doses of a new drug in a few patients. The phase 0 studies help researchers find out whether the drugs do what they are expected to do.

Does the treatment work?

If a new treatment is found to be reasonably safe in phase I clinical trials, it can then be tested in a phase II clinical trial to look for evidence that it works. The medicine is tested on a larger group of people who are ill. This is to get a better idea of its effects in the short term.

Post marketing surveillance.

Phase four trials take place after all the previous stages have been passed and have been given marketing licences. The medicine can now be made available on prescription. The safety, side effects and effectiveness of the medicine continue to be studied while it's being used in practice.