Cannabis and driving : a review of the literature and commentary.

The report summarises current knowledge about the effects of cannabis on driving and accident risk based on a review of available literature published since 1994. Cannabis consumption is frequent and increasing. A particular risk group is young males, in whom alcohol consumption is also common. This group is an a priori risk group for traffic accidents. Understanding of the biological mechanism of action of cannabis is far from complete. However, the identification of cannobinoid receptors in the cerebral cortex and the discovery of an endogenous ligand, anandamide, has heightened research activitiy. Development of new drugs acting on this system are imminent and may have implications for treatment of conditions, which would otherwise compromise ability to drive e.g. motor diseases. The effects of cannabis on laboratory based tasks show clear impairment with respect to tracking ability, attention and other tasks of greater ecological validity i.e. real driving or simulator tasks. Indeed, compensatory effort can be invoked to offset impairment in the driving task. This compensatory effort may be one reason for the failure to implicate cannabis consumption as an accident risk factor. However, this claim is difficult to substantiate in the absence of any valid epidemiological estimates of accident risk. Specifically, 4-12% of accident fatalities have detected levels of cannabis. However, most studies report that the majority of fatal cases with detected levels of cannabis are confounded by alcohol - although this rate is lower in the UK. Moreover, accident risk cannot be derived in the absence of baseline data for non-fatal cases. Alcohol alone or in combination with cannabis, increases impairment, accident rate and accident responsibility. Dose equivalence estimates suggest that the legal limit of alcohol (0.08% BAC) corresponds to 11 ng/ml THC, in terms of impairment. Key research objectives based on the analysis of the limitations of current knowledge about the effects of cannabis on driving and accident risk are presented. (Author/publisher)