How cannabis can impact driving impairment


Teenagers all over the United States have been taught to not drink and drive, but ongoing legalization of cannabis can add a new wrinkle. An investigation examines just how much cannabis could impair driving.

The past decades have driven home the message of “don’t drink and drive” to many teenagers, but the ongoing legalization of Δ9-tetrahydrocannabinol (THC) as well as increasing use of cannabidiol, which can be found in everything from gummy candy to face creams, has created a new potential hazard. An investigation in JAMA looks at how much the substances impact driving impairment following vaporization.1

The investigators ran a double-blind, within-participants, randomized clinical trial at the Faculty of Psychology and Neuroscience at Maastricht University in the Netherlands. The participants were all healthy occasional users of cannabis. They vaporized THC-dominant, cannbidiol-dominant, THC/cannabidiol-equivalent, and placebo cannabis. The doses of THC and cannabidiol were 13.75 mg. Primary end point of the research was standard deviation of lateral position (SDLP), a measure of lane weaving, during 100 km, on-road tests, which started at 40 and 240 minutes following cannabis use.

There were 26 participants who were randomized and 22 were able to complete all 8 of the driving tests. At 40 to 100 minutes after consumption, the SDLP was 18.21 cm with cannabidiol-dominant cannabis, 20.59 cm with THC-dominant cannabis, 21.09 cm with THC/cannabidiol-equivalent cannabis, and 18.28 cm with placebo cannabis. The researchers found that SDLP was appreciably more with THC-dominant cannabis (+2.33 cm [95% CI, 0.80 to 3.86]; P < .001) and THC/cannabidiol-equivalent cannabis (+2.83 cm [95% CI, 1.28 to 4.39]; P < .001) when compared to placebo cannabis. This was not true for cannabidiol-dominant cannabis (−0.05 cm [95% CI, −1.49 to 1.39]; P > .99). When looking at the SDLP at 240 to 300 minutes after consumption, it was 19.03 cm with cannabidiol-dominant cannabis, 19.88 cm with THC-dominant cannabis, 20.59 cm with THC/cannbidiol-equivalent cannabis, and 19.37 cm with placebo cannabis. When compared to the placebo, the SDLP was not significantly different for cannabidiol (−0.34 cm [95% CI, −1.77 to 1.10]; P > .99), THC (0.51 cm [95% CI, −1.01 to 2.02]; P > .99) or THC/cannabidiol (1.22 cm [95% CI, −0.29 to 2.72]; P = .20). Sixteen of the drives were not completed because of safety concerns.

The researchers concluded that THC and THC-cannabidiol cannabis did lead to greater impairment at 40 and 100 minutes after use, but the effect was not found if vaporization had occurred significantly earlier. Additionally, no difference was noted with cannabidiol-dominant cannabis. They did note that the dose tested may not be what is commonly consumed.


1. Arkell T, Vinckenbosch F, Kevin R, Theunissen E, McGregor I, Ramaekers J. Effect of cannabidiol and Δ9-tetrahydrocannabinol on driving performance: a randomized clinical trial. JAMA. 2020;324(21):2177-2186. doi:10.1001/jama.2020.21218

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