Phases of clinical trials

This page is about the different phases of clinical trials. It has information about

What trial phases are
Trial phases at a glance
Phase 0 trials
Phase 1 trials
Phase 2 trials
Phase 3 trials
Phase 4 trials
Trials covering more than one phase 

What trial phases are

Clinical trials testing new treatments are divided into different stages, called phases. The earliest phase trials may look at whether a drug is safe or the side effects it causes. Later phase trials aim to test whether a new treatment is better than existing treatments.

There are 3 main phases of clinical trials – phases 1 to 3. Phase 1 trials are the earliest phase trials and phase 3 are later phase trials. 

Some trials have an earlier stage called phase 0, and there are some phase 4 trials done after a drug has been licensed.

Some trials are randomised. This means the people taking part are put into one of treatment groups at random. Doing this means the results are more reliable.

Trial phases at a glance

Phase Number of people
taking part

 
Cancer type Main aims of trial Is it 
randomised?

 
0 Small – often about
10 to 20 people
Often lots of
cancer types
Testing a low dose of
the treatment to check it isn’t harmful
No
1 Small – often about
20 to 50 people
Often lots of
cancer types
Finding out about side
effects, and what
happens to the
treatment in the body
No
2 Medium - tens of
people, sometimes
over 100
Usually one or two
cancer types,
sometimes more
Finding out more about
side effects and looking
at how well the
treatment works
Sometimes
3 Large - hundreds or
thousands of people
Usually one cancer
type, sometimes more
Comparing the new
treatment to the
standard treatment
Usually
4 Medium to large,
variable
Usually one cancer
type, sometimes more
Finding out more about
long term benefits and
side effects 
No

Phase 0 trials

Phase 1 trials are usually the earliest trials of drugs in people. But your doctor might ask if you would like to join a phase 0 study. These studies aim to find out if a drug behaves in the way researchers expect it to from their laboratory studies.
 
Phase 0 studies usually only involve a small number of people and they only have a very small dose of a drug. The dose of the drug is too small to treat your cancer, but you are also less likely to have side effects.
 
Phase 0 trials aim to find out things such as:
  • whether the drug reaches the cancer cells
  • what happens to the drug in the body
  • how cancer cells in the body respond to the drug
You might have extra scans and give extra samples of blood and cancer tissue (biopsies) to help the researchers work out what is happening.
 

Phase 1 trial

Phase 1 is sometimes written as phase I. They are usually small trials, recruiting only a few patients. The trial may be open to people with any type of advanced cancer, usually those who have already had all other available treatments.
 
Phase 1 trials aim to find out:
  • how much of the drug is safe to give
  • what the side effects are
  • how the body gets rid the of drug
  • if the treatment helps shrink the cancer
Patients are recruited very slowly onto phase 1 trials. So even though they don't recruit many people, they can take a long time to complete. 
 
They are often dose escalation studies. This means that the first few patients that 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 side effects people have and how they feel, until they find the best dose.
 
In a phase 1 trial you may have lots of blood tests because the researchers look at how your body copes with and gets rid of the drug. They carefully record any side effects you may have and when you have them.
 
The main aim of phase 1 trials is to find out about doses and side effects. They need to do this first, before testing the potential new treatment to see if it works. Some people taking part may benefit from the new treatment, but many won't.
 

Phase 2 trials

Phase 2 is sometimes written as phase II. Not all treatments tested in a phase 1 trial make it to a phase 2 trial.
 
These trials can be for people who all have the same type of cancer, or for people who have different types of cancer.
 
Phase 2 trials aim to find out:
  • if the new treatment works well enough to be tested 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
These treatments have been tested in phase 1 trials, but you may still have side effects that the doctors don't know about. Treatments can affect people in different ways.
 
Phase 2 trials are usually 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).
 
Some phase 2 trials are randomised. This means the researchers put the people taking part into treatment groups at random. Find out about randomised trials.
 

Phase 3 trials

Phase 3 is sometimes written as phase III. These trials compare new treatments with the best currently available treatment (the standard treatment).
 
Phase 3 trials aim to find out:
  • which treatment works better for a particular type of cancer
  • more about the side effects
  • how the treatment affects people’s quality of life
They may compare standard treatment with:
  • a completely new treatment 
  • different doses of the same treatment
  • having the same treatment more, or less, often
  • a new way of giving a standard treatment (radiotherapy for example)
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.
 
Sometimes phase 3 trials involve thousands of people in many different hospitals and even different countries. Most phase 3 trials are randomised. This means the people taking part are put into treatment groups at random. See our information about randomised trials.
 

Phase 4 trials

Phase 4 is sometimes written as phase IV. These trials are done after a drug has been shown to work and has been licenced.
 
Phase 4 trials aim 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
We have information about how drugs are licensed.
 

Trials covering more than one phase

Most trials are just one phase. But some trials cover more than one phase. For example, the same trial can include both phase 1 and phase 2. So you may see trials written as phase 1/2 or phase 2/3.
 

Related information

 
 
 

Last reviewed

Please note - unless we state otherwise in the summary, you need to talk to your doctor about joining a trial.

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