Design
Cohort study linking three Norwegian administrative registries using unique person identifiers.
Setting
Information was retrieved from the Norwegian Prescription Database on any prescriptions ever received by the individuals for methadone and all prescriptions for benzodiazepines. The Norwegian Road Accident Registry provided information about motor vehicle accidents involving personal injuries on Norwegian roads. The Central Population Registry provided demographic information on all residents in Norway.
Participants
All Norwegian adults 18-69 years of age were observed for 2.5 years generating 8.1 million observed person years. Measurements: Standardized incidence ratio (SIR) was calculated by comparing the incidence of traffic accidents with personal injuries in patients exposed to methadone with the incidence in those not exposed.
Findings
During the 4626 person years observed in patients exposed to methadone, there were 26 motor vehicle accidents. There were very few accidents among the females who received methadone and they had no increased risk of being involved in motor vehicle accidents (SIR 1.1; 95 % CI 0.2-3.1). We observed an increased risk of involvement in accidents among males (SIR 2.4; 1.5-3.6). This figure did not change significantly when exposure to benzodiazepines was excluded.
Conclusions
Men exposed to methadone appear to have an increased risk of being involved in motor vehicle accidents involving personal injuries. This increased risk could not be explained by exposure of benzodiazepines.