Objective This study investigated the relationship between the number of different medications dispensed (NDMD) to senior drivers and the risk of injurious road traffic crashes (RTCs).

Design A matched case-control study with data from various population-based national registers was conducted. Cases were drivers aged 50–80 years involved in a crash in Sweden between 2005 and 2009. Only the first non-alcohol-related RTC was studied. Controls were residents with a valid license who did not crash. Four controls were matched by sex, age (year and month of birth), and place of residence. Exposure to NDMD prior to the crash date was assessed using four time periods: 1–8, 1–15, 1–30 and 1–90 days. Conditional logistic regression was used and analyses adjusted for civil status, occupation and dispensation of medications affecting the cardiovascular or nervous systems (C/N).

Results ORs (95% CI) increased progressively with the NDMD. For 1–8 days the OR ranged from 1.15 (1.10 to 1.20) for 1–2 medications to 1.27 (1.13 to 1.42) for five or more medications. The magnitude of the effect declined gradually with longer exposure periods, but remained when five or more medications were used. Adjusting for C/N medications resulted in slightly higher effects; for 1–8 days it ranged from 1.16 (1.10 to 1.23) for 1–2 medications to 1.35 (1.17 to 1.56) for five or more, with similar trends by exposure period. The highest effects were seen for single crashes and for drivers aged 66–80 years.

Conclusions The NDMD was linked to the likelihood of a senior driver being involved in an injurious RTC. The strength of the association steadily increased with increased NDMD, especially when medications were taken closer to the index date, or when more than five medications were dispensed.