Clinical trials & medical research

Side effects when taking part in trials

Clinical trials are carefully designed to be as safe as possible for everyone taking part, whichever treatment they receive. Some trials will involve very little risk. In others there may be some side effects from treatments, and finding out more about these side effects is one of the reasons we have trials. Researchers should explain to you what they already know about possible side effects to help you decide whether or not you want to take part. This will depend very much on the type of treatment being tested, so you will need to ask questions about the specific trial. Of course, it can be worrying hearing about all the possible side effects. However, it is important to bear in mind that not everyone will experience them. It is also important to remember that some side effects may just be the result of treatment (such as chemotherapy for cancer) and not specifically because of the trial.

Most young people we talked to said that possible known side effects of treatment were explained to them and they were able to ask questions and raise any concerns. (See also ‘Being invited to take part in a clinical trial: information and questions’.)

However, others felt that information they had received at the start of the trial ‘downplayed’ some of the side effects they experienced during the trial. 

Of course health professionals do not want to scare people about rare side effects that may never happen, but Katie pointed out it can actually be reassuring to know about all known side effects.

One of the main reasons for carrying out trials is to find out about side effects and measure how serious they are. Young people who take part in trials are monitored carefully. They will often require regular tests and may be asked questions about how they feel. 

It is important to report any side effects promptly as this tells the researchers something important about treatments being tested. They will be able to help manage the side effects, or advise you whether you should stop taking part. Some of these may be new side effects that were not expected. 

Alexander, aged 18, talks about his experience of taking part in a randomised placebo controlled trial on a new treatment for arthritis. When he first took part he had problems sleeping, which turned out to be a new side effect. 

Because neither Alexander nor the doctors and those involved in his care know which drug he is taking, it was sometimes a bit worrying. 

Sometimes it is not always clear whether symptoms are a result of the trial or just coincidence, but it is better to tell the doctors. Withdrawing from a trial because of side effects is entirely reasonable and no-one should feel they have to stay in for the sake of the trial or that they have failed if they pull out. (See also ‘Withdrawing from a trial’.)

Sometimes trials that do not involve drugs can also have side effects, for example minor surgery or tests that may leave you feeling uncomfortable. Several young people said they found blood tests and injections difficult. This is discussed in ‘What is involved in a trial: appointments and monitoring’. 

Sometimes there may be side effects from procedures involved in the trial as well as from the treatment itself. Robert is taking part in a Phase 1 gene therapy trial for the treatment of cystic fibrosis and because it is a Phase 1 trial, there is little known about the therapy. As well as receiving a dose of gene therapy he also had two bronchoscopies as part of the process to monitor his lungs.

In all trials the treatment may cause side effects or problems that doctors cannot predict. This may happen more often when less is known about the treatment being tested. Eden is taking part in a randomised controlled trial of treatment for Hodgkin’s lymphoma that involves taking four different types of chemotherapy. She experienced many side effects from the treatment, but one of the drugs was causing unexpected side effects.

Alexander also experienced side effects. Some of these were explained in the information he received at the start of the trial, but some were unexpected. 


Last reviewed March 2017.
Last updated March 2017.

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