To the Editor: In an editorial in the 3 April 2006 issue of the Journal,1 Roughead and Lexchin estimated the annual incidence of adverse drug events (ADEs) in patients presenting to general practitioners, based on our data presented in the same issue.2

Calculating the incidence of ADEs from general practice encounter data is fraught with difficulties. Roughead and Lexchin’s calculation depends on all general practice patients having an equal chance of being in the sample. This would only hold true if all patients attended their GP an equal number of times. However, in our study, the age distribution of patients with ADEs shows that they are more likely to belong to older patient groups with a much higher than average general practice attendance rate. They thus represent a smaller proportion of all general practice patients, as their chance of being in the sample is much higher.

Adjusting for the age and sex distribution results in an estimated incidence of about 1.6 million people. Further, these 1.6 million would, if asked, have reported an ADE in the previous 6 months. This cannot be extrapolated to an annual incidence of ADEs. The annual incidence figure could in fact be larger than that suggested by Roughead and Lexchin.

We believe that recurrent monitoring of ADEs in patients attending general practice is a useful way of measuring the impact of the interventions suggested by Roughead and Lexchin, regardless of the difficulty of extrapolating to population incidence.