How to read a paper on the short-term impairing effects of cannabis: A selective and critical review of the literature

Abstract

The prosecution of cannabis-presence driving offences (in the absence of any behavioural evidence of impairment) is ultimately based on the assumption that there is a tight causal relationship between positive toxicology for cannabis and impairment. The main purpose of this review is to examine the evidence for that relationship. We show that most experimental studies have failed to elicit statistically-significant cannabis-induced impairments for many of their possible outcomes. And many studies failed to demonstrate any impairment at all in regular users of cannabis (because of the development of tolerance). We argue that selective reporting by researchers, editors and the media has created the false impression that the evidence for cannabis-induced impairment is strong and consistent. Human beings are ‘over-engineered’ for the psychomotor skills required to drive safely. A benchmark level of cannabis-induced impairment is therefore required to distinguish unproblematic from ‘real-world’ impairment. The conventional benchmarks of statistical significance, effect size and BAC-equivalence are shown to be inadequate. However, a benchmark in terms of 30 years of normal cognitive aging has good face validity. The recent use of cannabis is indicated toxicologically by the presence of delta-9-tetrahydrocannabinol (THC) in blood or oral fluid. Evidence is provided that most THC-positive drivers are not impaired, and certainly not meaningfully impaired. It follows that the justice of stand-alone cannabis-presence driving offences must be questioned.

Does the co-use of cannabis exacerbate the effect of alcohol on the risk of crashing? A systematic review and meta-analysis

“…The degree of impairment resulting from cannabis use is clearly related to the dosage administered (e.g., Schlienz et al, 2020). However, that relationship does not seem to translate into demonstrable dose-response relationship between the concentration of THC in the blood or oral fluid and either the level of impairment (Arkell et al, 2021; White and Burns, 2022b: 16-18; Wurz and DeGregorio, 2022) or the risk of crashing (Macdonald, 2018: 96; White and Burns, 2021: 12-13). Nevertheless, it is certainly possible to get so intoxicated on cannabis that one is incapable of safely driving a car.…”

Blowing in the Wind: Cannabis Legalization, Insiders, and Methodological Insights from British Columbia

“…However, they note that skewed, agenda-driven reporting has undermined this area of inquiry. Finally, in a recent analysis of experimental studies on cannabis impairment and driving, White and Burns (2022) found that many of the existing studies have "failed to demonstrate any impairment at all in regular users of cannabis" (pg. 1).…”

Road safety implications of the partial legalisation of cannabis in Germany: protocol for a quasi-experimental study

“…6 Driving a vehicle immediately after cannabis use is associated with a low to medium increased risk of crashes 8 due to impaired cognitive abilities. [9][10][11] However, it should be noted that the evidence is found to be inconsistent, 12 possibly due to methodological biases, compensatory mechanisms and development of tolerance among some drivers. [12][13][14] Both CUD and (self-reported) DUIC are especially…”

“…[9][10][11] However, it should be noted that the evidence is found to be inconsistent, 12 possibly due to methodological biases, compensatory mechanisms and development of tolerance among some drivers. [12][13][14] Both CUD and (self-reported) DUIC are especially…”