Patients could stop taking their medicine if the risks of possible side effects are not clearly explained – says a study published in the British Journal of Health Pyschology.

Unclear medicine leaflets lead patients to overestimate the risk of side effects by up to 50 per cent – and can influence their decision about whether to take prescribed medication.

Researchers questioned 285 visitors to Cancer Research UK’s patient information website,

They found that people interpreted the risk of having side effects differently, when percentages were used as in: ’20 per cent of patients will have side effects’ or when words were used such as ‘common’, ‘very common’ or ‘uncommon’.

But they were much more likely to have an accurate understanding of the risks when they were described as ‘one in five people may have side effects’.

Previous studies have shown that patients want straightforward information about their treatment. They are most likely to read leaflets about their medicine when it is first prescribed to them. The risk of side effects will have a big influence on whether they take their medication.

Each patient will react differently to medication and until now it has been unclear how best to explain the risks.

Dr Peter Knapp, study author based at the University of Leeds**, said: “These are important findings which will help health workers and pharmaceutical companies explain information to patients in the best possible way.

“Giving information about medication in a confusing way could be dangerous. We need to explain risk in a way patients can relate to. For most people it is better to say ‘three in 10 people will have side effects’ rather than ’30 per cent’. Percentages are abstract concepts that some people will not understand. The use of percentages means that some people will misinterpret information or ignore it. Both could lead to them making an uninformed decision not to take a medicine.”

Liz Woolf, head of CancerHelp UK, said: “This study confirms what we know from producing content for CancerHelp UK – that you have to make sure you give information to people in a way they will understand it. Most people think side effects from drugs are much more common than they actually are. The study confirms that natural frequency – one in five – is the clearest way to express this type of information so that the majority will understand it clearly.”


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*Peter Knapp et al. Percieved risk of medicine side effects in users of a patient information web site: A study of the use of verbal descriptors, percentages and natural frequencies. British Journal of Health Psychology 2009; 14: 579-594. (Published by the British Psychological Society).

** Researchers from both the University of Leeds and the University of Bath took part in the study.