How do clinical trials work?

Drugs and treatments are usually assessed in three phases before they can be considered as a standard treatment option. Each clinical trial phase is designed to answer a specific set of questions about a new drug or treatment, which means there are strict guidelines about who can participate.

Phase I

The aim of a phase I trial is to find a safe dose of the treatment and look at the possible side effects. If the treatment is safe it will go into phase II to see if it has a positive effect on the patient.

In many cases, this will be the first time the treatment has been tested outside the laboratory. A small number of people – around 30 or fewer – who have no standard treatment options available, are chosen to participate.

Trials in this phase are often known as dose escalation studies. This means that at the start of the trial the first few participants will receive a very low dose of the drug. As more information about the safety of the drug becomes known the next group of patients will receive slightly higher doses. Researchers will monitor the wellbeing of the patients to determine the correct dose of the drug.

Some people in phase I studies will benefit from the new drug but many won't.

Phase II

This builds on the findings from phase I, improving knowledge of the potential side effects of the treatment and the best dose of the treatment to give.

It usually involves up to 100 people and looks at whether the treatment is having a positive effect on the type of cancer being studied. If tumours respond to the treatment (either by slowing down growth or reducing in size), it moves to phase III.

Phase III

This phase recruits the largest number of participants with hundreds or even thousands of people taking part, often from hospitals across the country.

It compares the new treatment with a standard treatment and will look to see if the new treatment improves overall survival or progression free survival (how long someone stays free of cancer).

Phase IV

This happens after a license to use the drug in normal clinical practice has been granted, and involves studies to monitor the medicine on an ongoing basis to see if there are any unexpected side effects, or if it causes problems in certain categories of people.

Trials covering more than one phase

Occasionally you may see trials written as phase I/II or phase II/III. This means that that trial covers more than one phase. Phase I/II indicates that the trial covers both phase I and phase II.