A roadside survey of drugs and driving was performed in south-eastern Norway in 2005-6. Samples of saliva from a total of 10,503 drivers above 20 years of age were analysed, and the results were weighted for under-and over-sampling compared to the population distribution in the study area. Weighted results were compared with data on dispensed prescriptions of zopiclone, codeine and diazepam at Norwegian pharma-cies in the same area and with self-reported use of cannabis. When using roadside data to estimate drug use, the use of medicinal drugs was under-estimated by 17-59% compared to amounts dispensed. One of the main reasons for the under-estimation may be that a large proportion of the users of psychoactive medi-cinal drugs are not frequent drivers. For cannabis, self-reported data corresponded approximately to the estimated prevalence range. The results indicate that roadside surveys cannot be used for accurate estima-tions of drug use in the population, but may provide minimum figures.