What is a clinical trial?

Cancer clinical trials are carefully designed research studies that investigate a new test or treatment for cancer in people. Trials may look at whether a treatment is safe, its side effects or how well a treatment or procedure works. Some trials look at how well treatments control symptoms or whether they improve quality of life.

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A clinical trial is a research study that tests different ways of improving people's health. A clinical trial aims to find out if a new treatment or procedure is safe and effective. New drugs are first tested in the laboratory before being tested on people.

In cancer research, clinical trials can answer questions about how to:

  • prevent cancer, for example using diet, exercise or drugs
  • determine any genetic predisposition to cancer
  • detect cancer at an early and treatable stage, for example using screening blood tests, scans or other special tests
  • diagnose cancer, for example using blood tests, scans or new technologies
  • treat cancer, for example using surgery, drugs, and radiotherapy or different combinations of treatment
  • control symptoms of cancer or its treatment, for example using drugs and 'complementary' therapies such as diet, vitamins, exercise, yoga, meditation, hypnotherapy or acupuncture.

Clinical trials are an important way to improve treatment for people with cancer. Trials are the only reliable way that doctors and patients can find out the effects of different treatments. Trials tell us if a new treatment is more effective than the current standard treatment and they also identify risks and side effects. A new treatment will only become the new standard after it has been proved effective and safe in clinical trials, and shown to be better than other treatments.

The results of clinical trials today will help people with cancer in the future. Participating in a clinical trial may or may not be of direct benefit to you. Many of the effective treatments that are used today are the outcomes of studies in past years.

Treatment trials

Treatment trials are the most common type of cancer clinical trial. They test new treatments and/or compare them with standard treatments. The treatments tested include:

  • drugs - such as chemotherapy, hormones, antibiotics or targeted drugs
  • radiation therapy
  • surgery
  • other treatments, such as high dose treatment with bone marrow support
  • palliative care - treatment that helps the symptoms of cancer
  • counseling and psychological support
  • complementary and alternative therapies
  • ways to combine treatments

Screening trials

The aim of screening is to detect cancer before it has caused symptoms, with the hope of offering early treatment and improved health outcomes. A screening test often does not diagnose the cancer directly, but if the test is abnormal (positive) it suggests the person is more likely to have cancer than if the test is normal (negative). People with abnormal tests will go on to have more diagnostic tests and, if needed, appropriate treatment. 

A screening trial may target either the general population, or a specific group at high risk of developing a particular cancer. Examples of screening include mammograms to detect early breast cancer, and faecal occult blood tests to detect early bowel cancer. 

Prevention trials

Prevention trials involve people who do not have cancer. The trials look to see whether a particular intervention stops cancer from developing. Prevention trials might be carried out on the general population or on a group that it is at high risk of developing cancer. For some cancers the intervention tested might investigate a drug, a new diet, weight loss, exercise, quitting smoking or limiting alcohol. 

Quality of life studies

A quality of life study looks at how the disease or treatment affects you. They are usually included as part of Phase 3 trials. Questionnaires are used to assess your quality of life. The type of questions that may be asked include whether you are more or less tired than usual, experiencing treatment related side effects, more or less anxious or depressed, how you are managing at home and at work and how you are feeling generally. It is important to know not only how a treatment affects the cancer but also how it makes you feel. This information can help inform others about the possible side effects of treatment.

Before a new treatment can become part of standard treatment it needs to go through a series of testing in what are called 'phases'. Following on from trials using animals, the main aim of early phase trials is to test if the treatment is safe for humans. Later phase trials investigate if the treatment is more effective than the currently available standard treatment. You may be asked to participate in any one of these phases of testing if you decide to take part in a clinical trial.   

Phase 0 trials

Phase 0 trials are the first-in-human studies for drugs that have been previously tested in animals. Usually a single drug dose is given to 10-15 people to gather preliminary data on how the drug is metabolised and what effect the drug has on the body. The dose of drug given is below what is thought to have a therapeutic effect.   

Phase 1 trials

These are the first tests of a treatment in humans and involve approximately 15-30 people. They aim to see if the trial treatment is safe and to find the best dose. The research team notes major and minor side effects. This helps researchers to work out the best dose and the best way to give the trial treatment. 

Phase 1 trials are usually only suitable for people with incurable cancer who have had all the known effective treatments for their cancer. People take part in these trials in the hope that the treatment may extend their life or improve their quality of life. Most often though, the reason to enter Phase 1 trials is to help people with cancer in the future.   

If a Phase 1 trial shows that the new treatment is safe it will go on to Phase 2 testing. 

Phase 2 trials

Phase 2 trials usually involve less than 100 people. The trials aim to see how well the new treatment works against cancer and to monitor for side effects. 

Phase 2 trials generally involve people who have already had cancer treatment. The chance of the treatment being tested in a Phase 2 trial being effective depends on the type and extent of the cancer and its response to previous treatment. 

If the new treatment shows promising effects (efficacy) against cancer and is safe then it is tested in Phase 3 trials. 

Phase 3 trials

Phase 3 trials involve from hundreds to thousands of people. The trials are usually conducted through doctors' offices, hospital clinics and cancer centres around the country or even around the world. They aim to test if the new treatment (the intervention treatment) is better than the current best standard treatment. They test treatments in real life settings, not the controlled circumstances under which Phase 1 and 2 trials are usually conducted. 

Participants in Phase 3 trials are put in either the new treatment group or the standard treatment group at random. People in the trial are monitored carefully to see the effect of the treatment on the cancer, and on the length and quality of life.   

If the new treatment works better, it may become the new standard treatment. In general, the new treatment needs to go through more than one Phase 3 trial for the treatment results to be accepted.   

Phase 4 trials

'Phase 4 trial' refers to the long-term monitoring that sometimes occurs after a drug has been licensed for use and put on the market. Phase 4 trials are to assess any long-term side effects of a new treatment. There are very few Phase 4 trials.

Randomised controlled trials are the best way to test if a new treatment is effective. In a randomised controlled trial people are allocated at random, using a process like a lottery, to receive the new treatment or a comparison or control treatment (usually the old treatment but sometimes a placebo).   This random allocation of people to treatments ensures the groups (new treatment group and control treatment group) are as similar as possible.   

It is important that any differences between the groups in the trial are due to the treatment, rather than the characteristics of the different groups. For example, one group of people could be older than the other at the start if groups were chosen by, say, location, and this might make the treatment results look better or worse, even if there was no real difference. 

Randomisation is used in all Phase 3 trials and some Phase 2 trials. These trials are called 'randomised controlled trials' or 'randomised clinical trials'. In these trials there are at least two different groups. One group, the 'intervention' group receives the treatment being tested and the other is the 'control' group. The control group gets the most widely-accepted current treatment, called the standard treatment. The people taking part in the trial are put into either one of these groups at random.   

A computer is usually used for randomisation. Personal and medical details (such as age, gender and the extent of cancer) may be entered into the computer first.   The computer gives each participant a code number. The code numbers are randomly allocated to the different treatment groups. The treatment arrives with a code number on it and is administered to the patient. 

'Blind' trials

Ideally, neither the study staff nor the participants should be aware of the treatment anyone is receiving. This is called a 'double-blind' study. The reason for 'blinding' is that knowledge of the treatment might affect the way a person behaves during the trial. It can also affect the way study staff respond to participants in a trial and affect their assessment of patients' symptoms and disease status.   

'Blinding' occurs at different points in the trial, so that neither the researchers recruiting participants nor the person assessing the outcomes of the trial knows which treatment group the participant belongs to. This is so everyone is totally objective when evaluating response to the treatment, without being influenced by the treatment group that the person is in. 

If only the participant is unaware of the treatment, it is called a 'single-blind' study.   


A placebo is an inactive (or fake) treatment designed to look like the treatment being tested. Placebos are rarely used in cancer treatment trials.   Occasionally a placebo will be used if there is no standard treatment for the control group to have. One group of people will receive the new treatment and another group will get the inactive treatment. Participants will not know which treatment they are getting ,so this will not unconsciously affect the results. 

The 'placebo effect' is a term used to describe an improvement in the condition of participants who think they are being treated, but are in fact getting an inactive treatment.