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Affiliation
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Department of Psychological Medicine, Christchurch School of Medicine and Health Sciences, PO Box 4345, Christchurch, New Zealand. E-mail: david.fergusson@chmeds.ac.nz |
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Abstract
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The paper by Blows and colleagues [1] on the effects of marijuana use on car crash injury and death risks provides a good illustration of the value of the case–control design in risk factor research. Although there is often advocacy for prospective or longitudinal studies of risk, the case–control design has a number of clear advantages in studying risk factors involved in rare but severe outcomes. These advantages include the ability to study sufficient numbers of cases of the outcome and the linking of assessments to a specific incident and set of circumstances, and are clearly evident in the research reported by Blows et al.
Their findings add to a growing literature that is beginning to explore the extent to which the use of cannabis has adverse consequences on driver behaviours. Their results show that while the habitual use of cannabis is associated with clear increases in the risk of death or injury from car crashes (OR = 9.5; 95% CI = 2.8–32.3), the association with acute exposure was non‐significant (OR = 0.8; 95% CI = 0.2–3.3). Of course, these findings are subject to the usual methodological caveats relating to the use of self‐report and possible failure to control confounding. With regard to the analysis of acute effects, there is also a possibility that the association was ‘over‐controlled’ as a result of the authors’ controlling factors such as sleepiness and seat belt usage that may have been a consequence of cannabis use. If these factors are not included in the regression adjustment, the results suggest an odds ratio of 3.9 (95% CI = 1.2–12.9) for acute exposure.
The principal conclusions that the authors draw from their analysis is that the relationship between cannabis use car crashes is likely to be complex and may be confounded by such factors such as risky driving. They also conclude that targeting high‐risk marijuana use groups may be more cost‐effective than general population interventions such as random roadside testing. Although these conclusions follow from the authors’ results they beg some important issues.
Keywords: Cannabis impaired driving |