Postmarketing drug surveillance.

To the Editor.— We concur with Rossi et al 1 that alert physicians and others spontaneously reporting their suspicions of adverse drug reactions is critical to the discovery of rare effects of marketed drugs. Spontaneous reporting is an essential element of the multicomponent postmarketing surveillance systems established by pharmaceutical manufacturers and the Food and Drug Administration. However, we disagree with the assumption that the reason for conducting systematic postmarketing drug surveillance is to identify unusual or unexpected adverse effects. The purpose of systematic surveillance is to quantify the occurrence of both beneficial and adverse effects, which is not possible with spontaneous reporting systems. Even expected effects that are occurring with reasonable frequency need to be quantified, particularly if they occur with a different frequency among previously unidentified subgroups of users. Furthermore, the failure of adversity to appear at a statistically significant level in the observation of 10,000 persons does not