Medical Technology

Driving high has seen a dramatic increase since the legalization of cannabis

Since Canada legalized marijuana in 2018, there has been a dramatic increase in the number individuals driving while high, new research shows.

Investigators studied over 4000 drivers treated after a motor vehicle collision in British Columbia trauma centers and found that before cannabis was legalized, a THC level greater than 0 ng/ml in the blood was present in roughly 10% of drivers. After the drug was legalized this percentage increased to 18%. The percentages of injured drivers with at least 2 ng/ml, the Canadian legal limit, and at least 5 ng/ml more than doubled.

“It’s concerning that we’re seeing such a dramatic increase,” study investigator Jeffrey Brubacher, associate professor, Department of Emergency Medicine, University of British Columbia, Vancouver, Canada, said in a press release.

“There are serious risks associated with driving after cannabis use and our findings suggest more [work] is needed to deter this dangerous behavior in light of legalization,” he said.

The study was published online January 12 in the New England Journal of Medicine.

Impact of Legalization?

The investigators note that the Canadian government introduced a law aiming to prevent cannabis-impaired driving by establishing penalties and criminal charges for drivers found with a whole-blood THC level of 2 ng/ml, with more severe penalties for those with a THC level of >5 ng/ml or >2.5 ng/ml combined with a blood alcohol level of .05%.

Cannabis use is “associated with cognitive deficits and psychomotor impairment, and there is evidence that it increases the risk of motor vehicle crashes, especially at higher THC levels,” they note.

“I’m an emergency physician at Vancouver General Hospital’s trauma center. We’ve been measuring drug levels in injured drivers since 2013 here in British Columbia and, in particular, we’ve been measuring THC levels,” Brubacher told Medscape Medical News. “We thought it would be interesting and important to see what would happen after legalization.”

The investigators studied 4339 drivers — 3550 whose accident took place before legalization of cannabis, and 789 after legalization — who had been moderately injured in a motor vehicle collision and presented to four British Columbia trauma centers between January 2013 and March 2020.

“Moderately injured drivers had injuries or potential injuries significant enough to have blood work done as part of routine clinical care,” said Brubacher. Drivers included in the study had excess blood remaining after the clinical testing had been completed, which was then used for drug analysis.

Insufficient Laws

After legalization there was an increased prevalence of drivers with a THC level >0 ng/ml, a TCH level of at least 2 ng/ml, and a THC level of at least 5 ng/ml.

The largest increases in a THC level of ≥ 2 ng/ml were in drivers 50 years of age or older and among male drivers (adjusted prevalence ratios, 5.18, 95% CI, 2.49 – 10.78 and 2.44, 95% CI, 1.60 – 3.74, respectively).

“There were no significant changes in the prevalence of drivers testing positive for alcohol,” the authors report.

Table. Percentage of impaired, injured drivers before and after cannabis legalization in Canada.

THC level (ng/ml) Before legalization After legalization Adjusted prevalence ratio (95% CI)
> 0 9.2% 17.9% 1.33 (1.05 – 1.68)
≥ 2 3.8% 8.6% 2.29 (1.52 – 3.45)
≥ 5 1.1% 3.5% 2.05 (1.00 – 4.18)

Brubacher said the evidence suggests these new laws “are not enough to stop everyone from driving after using cannabis.”

The findings have implications for clinicians and patients and for policymakers, he said.

“My moderately conservative recommendations are that if you are going to smoke cannabis, wait at least 4 hours after smoking before you drive. Edibles last longer, and patients should wait least 8 hours after ingesting [edibles] before driving. And of course, if you continue to feel the effects of the THC, you should avoid driving altogether until the time has elapsed and you no longer feel any effects.”

Brubacher hopes policymakers will use the study’s findings to “design public information campaigns and enforcement measures that encourage drivers, especially older drivers, to separate cannabis use from driving.”

Additionally, “policymakers shouldn’t lose sight of drinking and driving because that’s an even bigger problem than the risk of driving under the influence of cannabis.”

Focus on Older Adults

Commenting for Medscape Medical News, Anees Bahji, MD, an International Collaborative Addiction Medicine research fellow at the British Columbia Centre on Substance Use, called the study “interesting and relevant.”

He raised several questions regarding the “correlation between the level of a substance in a person’s system and the degree of impairment.” For example, “does the same level of THC in the blood affect us all the same way? And to what extent do the levels detected at the time of the analysis correlate with the level in the person’s system at the time of driving?”

An additional consideration “is for individuals with cannabis use disorder and for those who have developed tolerance to the psychoactive effects of THC: does it affect their driving skills in the same way as someone who is cannabis naïve?” queried Bahji, a clinical assistant professor at the University of Calgary who was not involved with the study.

Also commenting for Medscape Medical News, Eric Sevigny, PhD, associate professor of criminal justice and criminology at Georgia State University, Atlanta, described it as a “well-designed study that adds yet another data point for considering appropriate road safety policy responses alongside ongoing cannabis liberalization.”

However, the findings, “cannot say much about whether cannabis legalization leads to an increase in cannabis-impaired driving, because current research finds little correlation between biological THC concentrations and driving performance,” said Sevigny, who was not involved with the study.

The finding of “higher THC prevalence among older adults is also relevant for road safety, as this population has a number of concomitant risk factors, such as cognitive decline and prescription drug use,” Sevigny added.

The study was supported by the Canadian Institutes of Health Research. Brubacher and Sevigny have disclosed no relevant financial relationships. The other authors’ disclosures are listed here. Bahji reports receiving research funding from the Canadian Institutes of Health Research, the Calgary Health Trust, the American Psychiatric Association, NIDA, and the University of Calgary.

N Engl J Med. Published online January 13, 2022. Abstract

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