In injured drivers, THC was linked to modestly higher culpability odds

Responsibility analysis was conducted on almost 10,000 drivers injured over a 10-year period in Victoria, Australia as a result of a vehicular collision and in whom comprehensive toxicology testing in blood was conducted. Multivariable logistic regression models were fitted to compute unadjusted (OR) and adjusted odds ratios (aOR). In the multivariable model, adjustments were made for gender, age group, vehicle type, and whether the crash occurred in metropolitan or rural areas. The psychoactive drug-free driver was the control group. Drivers with ethanol (alcohol) present at any concentration (>0.01 % BAC) confirmed the well-known large increases in OR that occur with rising blood ethanol concentration, with an aOR of 16.4 (95 %CI 11.1-24.2). Drivers positive to methamphetamine and no other psychoactive drug also showed a large increase in the odds of culpability (aOR 17.2, 95 %CI 10.2-29.2). Drivers with Δ9-tetrahydrocannabinol (THC) in blood showed a modest increase in odds when all concentrations (> 1 ng/mL) were assessed (aOR 1.5, 95 %CI 1.1-2.0) that trended to a possible increased aOR 2.1 (95 %CI 0.9-4.9) when concentrations were 10 ng/mL or higher. Benzodiazepines in drivers also showed a significant increase in odds (aOR1.8, 95 %CI 1.2-2.8), as well as antidepressants (aOR 1.3, 95 %CI 1.0-1.6). Opioids such as codeine and oxycodone showed no measurable increase in the odds of culpability, whereas combinations of ethanol with methamphetamine or THC gave aOR of 22.6 (95 %CI 3.0-168) and 20.0 (95 %CI 7.3-54.5), respectively, as well as drivers positive to both THC and methamphetamine (aOR 14.6, 95 % CI 5.9-36.4). Drivers positive to one or more illicit drug, or a larger selection of psychoactive drugs in the absence of ethanol both gave similarly large increase in odds (aOR 4.0, 95 %CI 3.3-5.0). This study contributes to a better understanding of risks associated with driving in the presence of common psychoactive drugs.