Clinical trials are studies that test the effect of an intervention – such as a medicine, procedure or lifestyle change – in humans.

Trials are categorised into different phases, which depend on the aims of the study and what we already know about the intervention. Most trials are classed as one of three main phases: phase 1, 2, or 3 (sometimes written in roman numerals I, II, and III).

What happens during the phases of a clinical trial?

Phase 1

A phase 1 trial is the first time a new treatment is tested in humans, usually in healthy volunteers but sometimes involving people with the target condition. Phase 1 trials are small, often less than 50 people, and they primarily aim to make sure that a medicine is safe, and that it can reach and act at the necessary part of the body. This evidence is needed before research can proceed to phase 2.

Phase 2

Phase 2 trials will often involve a larger group of volunteers, made up of people that the intervention eventually aims to benefit (in our case, people with Parkinson’s). These trials look at a medicine’s biological effect – whether it performs in the body as expected – and are a longer test of safety. Some phase 2 trials will split the participants into two groups and give the medicine to one half, and a placebo with no active ingredients to the other half. This allows comparison between the two groups to see if the medicine is really having effects. These trials are known as ‘placebo-controlled’ trials and should be carried out blind; neither the researchers nor the participants know who’s taking the real medicine until the end, with participants being randomly assigned to each group.

Phase 3

If phase 2 results are encouraging, a phase 3 trial can follow. This are larger and longer, run across several sites, and could involve people from different countries. Phase 3 trials are designed to test long-term safety and demonstrate the effectiveness of a medicine in patients. Given their size, they will also pick up on side effects that might affect particular groups of patients. They will compare a new treatment with, or in addition to, the current best treatment.