In a CMAJ news article1 Wanniarachige states that it is difficult to establish a dose–effect response between cannabis use and driving risk. A review of the literature by Ramaekers and colleagues2 concludes that the “degree of performance impairment observed in experimental studies after doses up to 300 μg/kg THC [tetrahydrocannabinol] were equivalent to the impairing effect of an alcohol dose producing a blood alcohol concentration ≥ 0.05 g/dl, the legal limit for driving under the influence in most European countries.”2 Ramaekers and colleagues note that “[s]ignificant performance impairment emerges at serum THC concentrations > 2 ng/ml and crash risk significantly increases at serum THC concentrations between 4–10 ng/ml.”2 They observe that “combined use of THC and alcohol produces severe driving impairment and sharply increases the risk of drivers’ accident culpability as compared to drug-free drivers even at low doses.”2
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A link exists between attention-deficit/hyperactivity disorder (ADHD) and driving risk.3 The Dunedin study presents data on the effects of chronic frequent cannabis use starting in the teen years, showing toxic effects on cognitive neurodevelopment, which produce irreversible clinical syndromes indistinguishable from ADHD, and apparently irreversible loss of IQ, even after discontinuation of cannabis.4 This merits careful study by those advising legislators about the risks of intoxicated driving.