Study Finds No Evidence of THC Residual Effects on Driving

In the last quarter of 2019, a group of researchers based in Toronto revealed findings from a study conducted to determine the effects and residual symptoms of cannabis use on drivers. The study, published 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 effects on young users’ driving performances, over ninety 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.

Following the participants' completion of 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 conclude their research, the authors summarized: “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.”

The study contributes to a much larger discussion on cannabis and public safety, however, 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.

If you have been charged with driving under the influence of drugs or alcohol or have any questions concerning this article or the law, please contact us.

  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.

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Originally published Jan. 10, 2019, at

Comments 5

Really interesting. We need studies like this to help paas laws to make it FEDERALLY legal...not just a state by state deal.

14.01.2020 21:01

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14.01.2020 21:56

The "powers that be" will just ignore studies like this and only cite flawed/bogus or studies conducted by cannabis opponents like our fascist AG here in my part of AZ.

16.01.2020 03:11