Peter Knapp and colleagues from the University of Leeds asked 285 people to estimate the risk of an adverse event occurring when taking one of two medicines.
The frequency with which the events typically occurred were either described using natural frequencies, such as 'in about 30 people in 100', or verbally, such as 'very common' or 'common'. Verbal descriptions are used on most UK drug packaging.
Verbal descriptions led people to overestimate the risk of some drug side-effects by 50 per cent, compared with natural frequency descriptions.
The researchers suggest that using natural frequencies would help patients make better informed health choices.