New, Occasional, and Frequent Use of Zolpidem or Zopiclone (Alone and in Combination) and the Risk of Injurious Road Traffic Crashes in Older Adult Drivers: A Population-Based Case–Control and Case-Crossover Study

BACKGROUND Previous studies on the effect of zolpidem or zopiclone use on the risk of road traffic crashes (RTCs) have shown mixed results. OBJECTIVE Our objective was to determine the association between zolpidem or zopiclone use (as separate drugs or combined) and the occurrence of injurious RTCs among older adult drivers. METHODS This was a population-based matched case-control and case-crossover study based on secondary data linked together from Swedish national registers. Cases were drivers aged 50-80 years involved in a vehicle crash resulting in injuries between January 2006 and December 2009 for the case-control study (n = 27,096) and from February 2006 to December 2009 for the case-crossover study (n = 26,586). For the first design, four controls were matched to each case by sex, age, and residential area, and exposure was categorized into new, occasional, and frequent use of zolpidem only, zopiclone only, and combined zolpidem and zopiclone. For the case-crossover study, newly dispensed zolpidem or zopiclone users were assessed during the 28 days prior to the crash and compared with an equally long control period using a 12-week washout period. Matched adjusted odds ratios (OR) were computed using conditional logistic regression. RESULTS Increased ORs for all users were observed. In the case-control study, the highest odds were seen among newly initiated zolpidem-only users involved in single-vehicle crashes (adjusted OR 2.27; 95% confidence interval [CI] 1.21-4.24), followed by frequent combined zolpidem and zopiclone users [adjusted OR 2.20; CI 1.21-4.00]. In the case-crossover, newly initiated treatment with zolpidem or zopiclone showed an increased risk that was highest in the 2 weeks after the start of the treatment (OR 2.66; 95% CI 1.04-6.81). CONCLUSIONS These results provide more compelling evidence for the role of zolpidem or zopiclone in the occurrence of RTCs among older adults, not only in frequent users, but also at the beginning of treatment.