Cannabis, even at low doses, affects cognitive and psychomotor abilities required for driving. However, the causal relationship between cannabis use and traffic injuries has not been definitely established. Previous observational studies (mainly case-control designs) only partially control the effect of potential confounders, such as concurrent use of other psychoactive drugs or psychological factors. Moreover, in case- control designs, it is difficult to obtain a valid control group of drivers from the same population where the cases were recruited. The case-crossover design, in which cases and controls are the same subjects in 2 different periods (hazard period and control period), is useful for assessing the effect of transient exposures on acute outcomes. In this design, the self-matching guarantees a valid control group as well as complete adjustment for all confounders that remain stable over time, such as personality traits, driving abilities, or physical limitations. The authors used a case-crossover design to estimate the transient effect of cannabis on the risk of unintentional driving-related injuries during 60 or 120 minutes after use. (Author/publisher)
Abstract