The Food and Drug Administration has collected spontaneous reports on adverse events (AE) from manufacturers and individuals. These data provide useful information on the safety of marketed drugs. Due to many unique characteristics of this reporting system, the information is difficult to evaluate. Incidence rates for specific adverse events and drug combinations cannot be estimated. However, reporting rates (number oi reports per market share) based on prescriptions can be computed. These reporting rates provide signals of serious adverse experience that may deserve mention. when the ratio of reporting rates is used for the comparison of two drugs