MSACL 2024 Abstract

Introduction: The relationship between cannabis use and driving impairment is complex because of the unique pharmacokinetic and pharmacodynamic properties of delta-9-tetrahydrocannabinol (THC). With ethanol there is a clear relationship between amount of alcohol consumed, blood concentrations, and effects on driving performance. With cannabis these relationships are much more complex. The relationship between blood THC concentrations and crash risk has not been established. While it is clear that THC can impair driving, there are still uncertainties regarding the universality of such impairment and its time course. A key question remains: how to best identify drivers who are impaired by cannabis?

This presentation focuses on the toxicology results from the University of California-San Diego’s Center for Medicinal Cannabis Research recently competed randomized placebo controlled trial evaluating the effect of smoked cannabis on driving performance.

Objectives: The first objective of this study was to determine the relationship between concentrations of THC (and related cannabinoids) and performance on a driving simulator. The second objective was to evaluate how various toxicology cutoff concentrations in both blood and oral fluid affected the classification of participants deemed impaired by the field sobriety tests (FSTs) examinations.

Methods: 191 regular cannabis users were randomized to smoke 700 mg of placebo, 5.9% or 13.4% THC cannabis in a double blind manner. Blood, oral fluid and breath samples were collected serially up until 5 hours after smoking. During the study period participants drove a driving simulator and were administered FSTs by trained drug recognition experts (N=11). FSTs consisted of a walk and turn, modified Romberg, lack of convergence, one leg stand, and finger to nose tests. OF was collected using the Quantisal device. Concentrations of THC and related cannabinoids were quantified using isotope dilution liquid chromatography with tandem mass spectrometry.

Correlation between driving performance (swerving and coherence) and THC concentrations in blood, OF, and breath were determined using Spearman’s rho. P-values were adjusted for multiple testing using the False Discovery Rate (FDR) method. P-values < 0.05 were considered significant.

Results: No correlation (p > 0.05 in all cases) was observed between blood, oral fluid or breath THC concentrations and standard deviation of lateral position (swerving) or car following (coherence) on the driving simulator at any of the time points studied. The rate of impaired FST performance was significantly higher in the THC group compared to the placebo group up to 188 minutes after smoking. Seventy-one minutes after smoking, FSTs classified 81% of the participants who received active drug as being impaired. However, 49% of participants who smoked placebo (controls) were also deemed impaired. OF showed less of an impact than blood for reclassifying the active drug cohort, while reclassifying a higher percentage of the placebo group as not impaired.

Conclusion: The complete lack of a relationship between the concentration of the centrally active component of cannabis in blood, OF, and breath is strong evidence against the use of per se laws for cannabis. In the largest randomized double-blinded placebo controlled trial to date, our data confirm that THC concentrations (and/or metabolites/related cannabinoids) in blood, OF, or breath cannot be used as a sole indicator of impairment.

Adding a requirement of a positive toxicology test in OF to the FST exam slightly decreased the percentage of participants who smoked active drug that were classified as possibly being under the influence of cannabis but dramatically decreased the percentage of placebo group subjects that were classified as such.