OBJECTIVE Pharmaceutical company drug information center (PCDIC) responses to queries about adverse drug events (ADEs) were studied to determine whether PCDICs search sources other than the prescribing information on the package insert (PI) and whether the PCDICs' approach differs according to whether an ADE is listed in the PI (labeled) or not (unlabeled). METHODS Companies were selected from a list of PCDICs in the Physicians' Desk Reference. One oral or injectable prescription drug from each company was selected. For each drug, a labeled ADE and an unlabeled ADE about which to query the PCDICs were randomly selected from the index of an annual publication on ADEs. The investigators telephoned the PCDICs with an open-ended inquiry about the incidence, timing, and management of the ADE as reported in the literature and the company's internal data; they clarified that the request did not concern a specific patient. Whether or not information was provided, the source searched was recorded (PI, literature, internal database), and the percentages of PCDICs that used each source for labeled and for unlabeled ADEs were analyzed. RESULTS Results were obtained from 100 companies to questions about 100 drugs (200 ADEs). For ADEs overall, 80% used the PI, 50% the medical literature, and 38% internal data. For labeled versus unlabeled ADEs, respectively, the PI was used by 84% and 76%; literature, both 50%; and internal data, 35% and 41%. The PCDIC specialists referencing the PI did not always provide accurate or up-to-date information. Some specialists, when asked to query internal databases, said that was not an option. CONCLUSIONS For both labeled and unlabeled ADEs, the PI was the primary source used by PCDICs to answer safety questions about their products, and internal data were the least-used source. Most resources used by PCDICs are readily available to practicing pharmacists.