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Find a clinical trial

Types of Clinical Trials


    Phase 4 Trials


    Phase 1 Trials


    Phase 2 Trials


    Phase 3 Trials

Phase 1 trials

These are the earliest trials in the life of a new drug or treatment. Phase 1 is sometimes written as Phase I. They are usually small trials, recruiting anything up to about 30 patients. The trial may be open to people with any type of cancer. When laboratory testing shows that a new treatment might help treat cancer, Phase 1 trials are carried out to establish:

  • The safe dose range
  • The side effects
  • How the body copes with the drug
  • If the treatment shrinks the cancer

Although Phase 1 trials don't recruit many patients, they can take a long time to complete. The first few patients to take part (called a 'cohort' or group) are given a very small dose of the drug. If all goes well, the next group have a slightly higher dose. The dose is gradually increased with each group. The researchers monitor the effect of the drug, until they find the best dose to give. This is called a dose escalation study.

In a Phase 1 trial, you may have several blood tests, to assess how the drug is affecting you. They also look at how your body copes with, and gets rid of the drug. They record any side effects.

People taking part in Phase 1 trials often have advanced cancer and have usually had all the treatment available to them. They may benefit from the new treatment in the trial, but many won't.

Phase 1 trials are important because they are the first step in finding new treatments for the future.

Phase 2 trials

Not all treatments tested in a Phase 1 trial make it to a Phase 2 trial. Phase 2 is sometimes written as Phase II. These trials may be for people who all have the same type of cancer, or who have several different types of cancer. Phase 2 trials aim to find out:

  • If the new treatment works well enough to test in a larger Phase 3 trial
  • Which types of cancer the treatment works for
  • More about side effects and how to manage them
  • More about the best dose to use

Although these treatments have been tested in Phase 1 trials, you may still have side effects that the doctors don't know about. Drugs can affect people in different ways.

Phase 2 trials are often larger than Phase 1. There may be up to 100 or so people taking part. Sometimes in a Phase 2 trial, a new treatment is compared with another treatment already in use, or with a dummy drug (placebo). If the results of Phase 2 trials show that a new treatment may be as good as existing treatment, or better, it then moves into Phase 3.

Phase 3 trials

These trials compare new treatments with the best currently available treatment (the standard treatment). Phase 3 is sometimes written as Phase III. These trials may compare:

  • A completely new treatment with the standard treatment
  • Different doses or ways of giving a standard treatment

Phase 3 trials usually involve many more patients than Phase 1 or 2. This is because differences in success rates may be small. So, the trial needs many patients to be able to show the difference.

For example, 6 out of 100 more people (6%) get a remission with a new treatment compared to standard treatment. If there were 50 people in the new treatment group and 50 people in the standard treatment group, there may be 3 more people in remission in the new treatment group. The 2 groups would not look that different. But if the researchers gave each treatment to 5,000 people, there could be 300 more remissions in the new treatment group. Sometimes Phase 3 trials involve thousands of patients in many different hospitals and even different countries.


Phase 3 trials are usually randomised. This means the researchers put the people taking part into 2 groups at random. One group gets the new treatment and the other the standard treatment.

Overview studies

Trial overviews are studies that combine all the results from Phase 3 trials of a new treatment. They are sometimes called meta-analyses. The idea is to get a broader picture of how well a treatment works. The more data (information) you have, the more accurate the results are likely to be.

Phase 4 trials

Phase 4 trials are done after a drug has been shown to work and has been granted a licence. Phase 4 is sometimes written as Phase IV. These trials look at drugs that are already available for doctors to prescribe, rather than new drugs that are still being developed. The main reasons for running Phase 4 trials are to find out:

  • More about the side effects and safety of the drug
  • What the long term risks and benefits are
  • How well the drug works when it’s used more widely than in clinical trials

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Sarah Cannon Research UK NHS Ventures
University College London Hospitals