Trends in alcohol, MDMA, methylamphetamine and THC in injured and deceased motor vehicle drivers and motorcyclists over a decade (2010-2019) in Victoria, Australia

Background: Driving under the influence of alcohol and other drugs contributes significantly to road traffic crashes worldwide. This study explored trends of alcohol, methylamphetamine (MA), 3,4-methylenedioxy-N-methylamphetamine (MDMA) and Δ9-tetrahydrocannabinol (THC), in road crashes from 2010 to 2019 in Victoria, Australia. Methods: We conducted a cross-sectional analysis using data from the Victorian Institute of Forensic Medicine and Victoria Police, examining proscribed drug detections in road crashes. Time series graphs per substance explored indicative trends and comparisons between road users. Negative binomial regression models, with robust SEs and adjusted for exposure (kilometres travelled, Victorian licence holders), modelled the incidence rate ratio, with a Bonferroni-adjusted α=0.007 for multiple comparisons. Results: There were 19 843 injured drivers and 1596 fatally injured drivers. MA had the highest prevalence (12.3% of fatalities and 9.1% of injured drivers), demonstrating an increase over time. Overall, 16.8% of car drivers and motorcyclists tested positive for one or more drugs, with 14% of crashes involving a blood alcohol concentration (BAC)≥0.05%. MA and THC were the most common drugs in fatalities. Between 2015 and 2019, MA was detected in 27.9% of motorcyclist fatalities, followed by THC (18.3%) and alcohol ≥0.05% (14.2%), with similar but lower frequencies among injured motorcyclists. Alcohol detections (≥0.05% BAC) in fatalities declined, but increased in injured motorcyclists and car drivers until plateauing in 2017. THC detections rose among injured drivers until 2018, detected in 8.1% and 15.2% of injured and fatal drivers, respectively. MDMA-positive driving decreased among injured drivers and remained stable at ∼1% of fatalities. Conclusions: Despite enhanced road safety measures in Victoria, drug-driving persists, indicating a need for revised prevention strategies targeting this growing issue.