Go to the page content

A clinical trial is a research study where volunteers agree to test a potential new medicine or a medical device.

The trial is designed to discover whether this potential new medicine or medical device works and it has a favourable risk benefit profile. It’s only through clinical trials and the help of volunteers like you that important questions about potential new medicines can be answered.

What are clinical trials for?

Clinical trials test potential new medicines to see:

  • If the medicine works
  • If there are safety concerns
  • What the side effects are
Are clinical trials safe?

Strict rules about how to run a clinical trial are in place to protect the rights, safety, privacy, and well-being of trial volunteers.

How are clinical trials set up?

Every clinical trial has a plan, also known as a protocol. The plan outlines the details of the trial, including which people may enrol, how long the trial will take, and how the results will be measured. Before the trial can start, a UK independent ethics committee and the Medicines and Healthcare products Regulatory Agency (MHRA) must approve this plan.

Keep in mind, clinical trials will only take place if:

  • There is a scientific and medical reason for the trial
  • The benefit/risk profile has been assessed as acceptable for the trial volunteers 

The 4 phases of clinical research

Before a potential new medicine can be tested in a clinical trial, it must first be tested in animals. Testing new medicines usually goes through the following 4 phases.

Phase I

Basic safety

These trials test a potential new medicine to better understand the safety profile. In this phase, a small number of people, who may be healthy, are given the medicine. When the safety profile of the new medicine has been established, the medicine can move to Phase II

Phase II

Does the medication work?

These trials test the potential new medicine on a larger group of people (usually a few hundred) with a specific disease for a longer time. This is done to see if or how the medicine works. This is also known as a medicine’s efficacy.

Phase III

Testing in a larger group

These trials test medicines in even larger groups of people (typically several hundred) with a specific disease. This phase compares the potential new medicine to the usual medicine used for the disease, or to a placebo (dummy medicine that doesn’t have active ingredients).

Doctors will often use the “blinded” method for Phase II and Phase III trials. This means that neither the participant nor the doctor will know whether it is the potential new medicine or the placebo that the participant receives.

Phase IV

Post-approval testing

In these trials, the medicine tested has already been approved for use. Phase IV trials include the largest group of participants (usually several hundred to thousands of volunteers). Phase IV trials are sometimes called Post Marketing Surveillance Trials.

Want to know more?

Find out more about what it may be like to participate in a clinical trial.

What it may be like to participate in a clinical trial

What is a clinical trial?

It's important to attend every clinical trial visit so that the trial team can complete the checks and tests needed to generate the trial results

What is a clinical trial?

You will be monitored closely and your team will be available for questions and support.

What is a clinical trial?

You may be able to join a trial at any phase.

What is a clinical trial?

Clinical trials are one of the most important steps to finding new medicines.

Questions or concerns

Do you have any question about what it’s like being a clinical trial volunteer? Have a look at our answers to frequently asked questions.



This site contains information about clinical trials sponsored by Novo Nordisk. It is not intended to replace the advice of a healthcare professional and should not be construed as providing advice or making a recommendation. The information on this site should not be relied on as the basis for any decision or action. Only a physician can determine whether a specific product is correct for a particular patient. If you have questions regarding any information contained on this site you should consult a physician.