Evaluation of Over-the-Counter Pharmaceutical Sales As a Possible Early Warning Indicator of Human Disease

urveillance of over-the-counter (OTC) pharmaceutical sales may someday be used to give public health offi cials an early warning about developing public health conditions. To acquire confi dence in this surveillance, it is necessary to establish fi rmly that OTC sales may be used to predict changes in clinical conditions with a useful time lead. This article reports the magnitude of correlations between some OTC sales data and clinical data and measures the time lead after controlling for day-of-week effects and some holiday effects. Results, derived for several subregions of the National Capital Area, indicate about a 90% correlation between fl u-remedy sales and physician diagnoses of acute respiratory conditions. The sales in question tend to occur approximately 3 days prior to the physician–patient encounters.