[The effect of the wording of information about side effects on patients' interpretation of reported side effects]

To determine whether changes in the side effects frequency information in a patient information leaflet affects the interpretation of the frequency, the recall of this information and the number of side effects reported.

Randomized controlled trial.

A field experiment was carried out in which three versions of a package insert concerning diclofenac were randomized and tested in 33, 30 and 34 patients respectively. Version 1 contained the original side effects information. In version 2 the frequency descriptors were manipulated (lowered, e.g. 'sometimes' was replaced by 'seldom'). This was because a preliminary investigation had shown that patients make higher assessments of side effect frequencies than the leaflet writers want to convey. In version 3 an introduction was added with general information on side effects.

The frequency assessments differed enormously between subjects. Especially the lower frequencies were virtually impossible to convey by means of words. The lower-ranking frequency descriptors did not only result in lower frequency assessments, but also led to lower recall score for side effects mentioned in the text and to a smaller number of side effects actually reported by patients. The only effect of the introduction was to lower the frequency assessments for the side effects following the introductory paragraph.

Side effects with high assessed frequencies were read with more attention. The advantages and disadvantages of numerical frequency statements need to be discussed.