Canadian Study Assesses THC Residual Effects on Driving

The Rodman Law Group, LLC

The Rodman Law Group, LLC


Reporting no evidence of residual effects on drivers, last fall’s findings contribute to the growing discussion of cannabis’s effects on consumers.

Over the autumn of 2019, a group of researchers out of Toronto announced findings from a study conducted to determine the impact and residual effects of cannabis use on drivers. The study, included in the December issue Drug and Alcohol Dependence, an international journal devoted to publishing original research, scholarly reviews, commentaries, and policy analyses in the area of drug, alcohol and tobacco use and dependence suggests that, although drivers do tend to decrease driving speed after smoking cannabis, the plant’s associated “high” does not significantly impair driving ability.

Initiated to determine the plant’s influence on young users’ driving performances, over ninety test subjects between the ages of 19 and 25 were selected to participate in the double-blind, randomized clinical trial. Of those studied, all consumed cannabis on a weekly basis. Subjects were given high-THC, low-THC, or non-THC placebo cigarettes and monitored 30 minutes, 24 hours, and 48 hours after cannabis inhalation.

After study participants completed a driving simulator, data revealed an evident spike in heart rate, visual analogue scale (“VAS”) drug effect, and drug high. However, symptoms were noted to gradually decrease over time. For those given THC, driving speed was noticeably slower than their placebo-administered counterparts.

To summarize their research, the authors concluded: “Acutely, cannabis caused decreased speed, increased heart rate, and increases in VAS drug effect and drug high. There was no evidence of residual effects on these measures over the two days following cannabis administration.”

While the study contributes to a much larger discussion on cannabis and public safety, much remains to be settled in determining the full effects of cannabis on consumer driving ability. The National Institute on Drug Abuse (“NIDA”) asserts that numerous studies have been performed suggesting that cannabis significantly increases the risk of being involved in an accident.1 A federal institution dedicated to researching drug use and abuse, NIDA’s remarks on driving under the influence of cannabis broadly attempt to identify negative effects of the substance on consumers, That said, NIDA’s overview of cannabis and driving also cites a 2015 study by the National Highway Traffic Safety Administration, in which, “no significant increased crash risk attributable to cannabis after controlling for drivers’ age, gender, race, and presence of alcohol.”2

Conversely, some studies create a further complicated picture. In a 2018 review of all 36 trials on human cannabis tolerance after recurring dosing, British researchers reported that frequency of use is crucial to assessing cannabis’s influence. Explaining the meta-analysis present in the published findings, the investigators posited:

Studies indicated relatively minor or no effects of repeated Δ9-THC administration in RU (regular users) on a number of cognitive domains including learning, memory, vigilance, and psychomotor ability. This absence of effect in RU might indicate the development of full tolerance.”3

The science of cannabis is developing rapidly, and new studies are published every year that advance our understanding of how the plant impacts daily users. There is still much to be learned, and there is a chasm between the science we know and the laws we follow. Studies like those from the Toronto researchers are a massive step in the right direction, as we work to mitigate this country’s draconian laws.

  1. National Institute on Drug Abuse. “Does Marijuana Use Affect Driving?” Marijuana. National Institutes of Health, September 2019.
  2. National Institute on Drug Abuse. “Does Marijuana Use Affect Driving?”
  3. NORML. “Meta-Analysis: Repeated Cannabis Exposure Associated With Reduced Impact On Cognitive, Psychomotor Performance.” NORML – Working to Reform Marijuana Laws. The National Organization for the Reform of Marijuana Laws, November 1, 2018.

DISCLAIMER: Because of the generality of this update, the information provided herein may not be applicable in all situations and should not be acted upon without specific legal advice based on particular situations.

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