Three-year review of tetrahydrocannabinol and alcohol blood concentrations and effects in impaired driving investigations in Ontario, Canada

Driving under the influence of cannabis is a significant public safety issue. Tetrahydrocannabinol (THC), the main psychoactive compound in cannabis, can impair driving ability and when used in combination with alcohol, the level of impairment can increase. Case studies documenting blood concentrations of THC and associated observations of impairment in individuals suspected of impaired driving continue to provide important contributions to the scientific literature. Retrospective analysis was performed using data from impaired driving investigations in which THC alone, and in combination with alcohol, were the only findings in blood samples submitted to the Centre of Forensic Sciences between 2021 and 2023, inclusive. In cases where only THC was present, blood concentrations ranged from 1.1 to 72 ng/mL (mean 12 ng/mL, median 9.5 ng/mL). When both THC and alcohol were present, the mean blood THC concentration was 13 ng/mL, and the mean blood alcohol concentration was 145 mg/100 mL. The most common driving and Drug Recognition Expert (DRE) observations included collisions, erratic driving, eyelid tremors, poor balance and coordination, and bloodshot eyes. Observations of driving impairment were more frequently observed in cases where both THC and alcohol were detected in the blood. These findings provide additional data to inform the interpretation of blood THC concentrations in the context of DRE observations and suspected impaired driving investigations.