Combined verbal and numerical expressions increase perceived risk of medicine side-effects: a randomized controlled...

The study evaluated European Medicines Agency (EMA) recommendations on communicating frequency information on side-effect risk.

The study used a 2 × 2 factorial trial, with random allocation of information about 10 side-effects of paclitaxel (Taxol) expressed using one of four formats. Recruitment was via the CancerHelpUK website. Information was conveyed using numerical frequency bands (e.g. 'may affect up to 1 in 10 people') or combined verbal terms and numerical bands (e.g. 'common: may affect up to 1 in 10 people'); in addition, the risk qualifier verb was manipulated, with risks expressed either as 'will affect…' or 'may affect…'. Participants then made six side-effect frequency estimates indicated their satisfaction with the information and evaluated the side-effects: how bad; how likely; how risky to health; and their influence on taking paclitaxel.

The sample comprised 339 people, of whom 37.5% had cancer. The combined verbal and numerical risk expressions resulted in higher estimates of side-effects, four of which reached statistical significance (P < 0.05), and participants also said that side-effects would be more likely. Use of 'may affect' or 'will affect' did not result in differences in any estimates.

This is the first evaluation of the full range of combined verbal and numerical risk expressions recommended in EMA guidance; it demonstrates that they can lead to significant risk overestimations when compared to numerical frequency bands alone. The EMA should consider revising its guidance. Government agencies and professional bodies should be cautious about recommendations for risk communication in the absence of empirical evidence.