Clinical Trials

Different types of clinical trial

Clinical trials cover a broad range of different types of research (see also our introductory explanation in ‘What are clinical trials and why do we need them?’) Trials are often used to test new medicines or vaccines but can also be used to look at new combinations of existing treatments or to test whether giving a treatment in a different way will make it more effective or reduce any side effects. Here Phil describes a trial comparing different medications for treating high blood pressure and cholesterol, and Wendy describes a trial comparing different chemotherapy regimens for bowel cancer.

Some trials are designed to try out ways to prevent a particular disease in people who have never had the disease, or to prevent a disease from returning. The treatments being tested in these types of studies can include vaccines, but may also involve drugs or dietary supplements such as vitamins and minerals. Amanda took part in a trial testing whether yoghurt could help prevent irritable bowel syndrome (see also ‘Blinded trials’). She is a public health doctor and has also developed proposals to test the effectiveness of selenium in preventing cancer.

Drug trials, especially in cancer, are probably the most familiar type of trial for many people, but clinical trials are not always about testing medicines. They can be used to test ‘interventions’ aimed at changing a person’s behaviour or lifestyle. This could include an educational programme designed to improve a person’s understanding of their medical condition and help them to manage it more effectively, or a psychological treatment for mental health problems.

The trial Jenny took part in was comparing the effectiveness of two different types of medication and an intra-uterine device (IUD or coil) as a way of reducing heavy periods.

Trials of different surgical procedures are not as common as drug trials, but they are becoming more frequent. Surgical trials may, for example, compare different types of surgery, or they may compare surgery with a non-surgical treatment.

Some trials are set up to evaluate screening and diagnostic tests, such as the UK Clinical Trial of Ovarian Cancer Screening. This aims to analyse whether either a blood test (for the tumour marker CA125) or an ultrasound scan are sufficiently accurate in detecting ovarian cancer to be worth offering to all women.

Iain Chalmers, one of the authors of the book 'Testing Treatments' (a web resource is available at, explains why screening tests may be problematic, using the example of prostate cancer. Further trials may be needed to reduce uncertainty about which treatment to offer once people have been screened and a problem detected. In prostate cancer, the trial is comparing very different types of intervention: active monitoring, surgery, and radiotherapy.

There is also growing interest in testing different ways of giving people health information, to see which is most helpful to them in making decisions or understanding and managing their condition.

If you are asked to take part in a trial, the UK Clinical Research Collaboration booklet on ‘Understanding Clinical Trials’ (see Resources), has a checklist of the kind of questions you might like to ask about how the trial is organised and what it involves. It also contains information on different types of trial and why they are needed.

Several people noted how important it is for trails to be well-designed, build on previous research, measure things that matter to patients, and include enough people to get reliable results. The number needed – the ‘sample size’ - will vary from trial to trial, depending on the condition, the treatment being tested and how big or small an effect the treatment is expected to have. The sample size is proposed by statisticians doing ‘power calculations’. In some cases a small number of participants may be enough, but as Sir Richard Doll explains below, if you are looking for fairly small changes in outcome you will need large numbers of participants to pick these up reliably.

See also ‘What are clinical trials and why do we need them?’ and ‘Under-researched topics/priorities for other research’.

Last reviewed September 2018.
Last updated September 2018.


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