Driving while stoned leads to more traffic accidents in a country where marijuana is legal, new study says

Originally Published: 06 SEP 23 11:00 ET

Updated: 06 SEP 23 13:36 ET

By Sandee LaMotte, CNN

(CNN) — Being stoned behind the wheel can be more dangerous than driving drunk in Canada, where recreational cannabis was legalized in 2018, according to a new study.

Documented marijuana-related traffic accidents that required treatment in an emergency room rose 475% between 2010 and 2021, the study found. Car crashes due to drunk driving grew only 9.4% during the same time period, although the raw numbers of alcohol-related accidents was in the thousands, not the hundreds as with cannabis.

“The concern is that the increase in these rare but very severe traffic injuries are capturing broader trends of increasing cannabis-impaired driving over time and after legalization,” said study author Dr. Daniel Myran, assistant professor of family medicine at the University of Ottawa.

Just after Canadian legalization in 2018, when marijuana stores and products were limited, researchers found a 94% increase in emergency room visits, Myran said. As commercialization increased and marijuana was more widely available, visits to the emergency room grew 233% compared to the period before recreational weed was legalized.

“The main message of this very well-conducted study is not the absolute number of crashes, but the increased rates. Cannabis is also probably under-reported in car crashes, and so the absolute number might be way higher,” said Dr. Marco Solmi, associate professor of psychiatry at the University of Ottawa and investigator at Ottawa Hospital Research Institute in Canada. He was not involved in the study.

Car crashes involving weed were serious. In marijuana-involved accidents, nearly 90% of the victims arrived by ambulance, the study found. When no alcohol or cannabis was involved, the number of people that required an ambulance dropped to 40%. In addition, nearly 50% of marijuana users in a car accident required hospital admission, compared to just over 6% of those who did not use.

Intensive care admissions were also higher. Nearly 22% of accidents involving those driving while stoned needed intensive care, compared to just less than 2% of crashes without alcohol or cannabis involvement, according to the study published Wednesday in the journal JAMA Network Open.

“Because of the way that cannabis impacts driving performance — it reduces reaction time, decreases the ability to focus or pay attention to multiple events, and may increase risk-taking behaviour — people who are cannabis-impaired while driving may be driving faster, noticing hazards later, and deaccelerating slower …  a recipe for getting into more severe traffic collisions and requiring higher levels of care,” Myran said in an email.

A wide-spread problem

While the new study was specific to Canada, the problem is happening around the world in areas where recreational cannabis use is legal, according to a recent “umbrella review” of more than 100 clinical trials and meta-analyses on the pros and cons of marijuana.

“The general perception of cannabis as a ‘natural’ harmless plant is probably misleading young subjects that end up consuming high THC products, with untoward events including car crashes,” said Solmi, who coauthored the review, via email.

THC stands for tetrahydrocannabinol, which is the part of the cannabis plant that produces a “high.”

“In addition to car crashes, persons using cannabis are at increased risk of poor cognitive performance — which might contribute to car crash and failing education — and a risk of mental disorders,” Solmi  said.

Driving under the influence of alcohol has been on the decline in the United States, but the last National Highway Traffic Safety Administration report from 2014 found a 48% increase in drivers testing positive for marijuana. An updated report from the NHTSA is expected in 2024.

The 2016 Drug and Alcohol Crash Risk Study found marijuana was the most frequently detected drug other than alcohol; in addition, people using cannabis were more likely to be involved in crashes.

Dealing with impaired drivers high on marijuana is a fact of everyday life in Colorado, the second US state to legalize recreational cannabis, said clinical pharmacologist Robert Page II, who was not involved with the new study. Page chaired the medical writing group for the American Heart Association’s 2020 scientific statement on marijuana.

“Here in Colorado, when I’m on the road, I assume everybody’s stoned,” Page said. “Compared to alcohol, which is a depressant, cannabis alters perception. A driver’s ability to react is delayed, and so therefore it can lead to accidents.”

Covid-19 may have worsened the problem, studies found. A 2020 study of seriously or fatally injured victims found the overall prevalence of alcohol, cannabinoids and opioids increased during the pandemic compared to before.

Misconceptions about safety

Many people don’t realize driving while stoned can be dangerous — they even see it as safe, Page said. Research shows those beliefs are clearly wrong. The THC in marijuana impairs psychomotor skills, impedes the ability to multitask, disrupts lane tracking and cognitive functions, and divides attention from the task at hand — driving — according to several studies.

Another issue is the rising potency of cannabis, Myran said. “Use of higher-strength products also increases impairment and risk. Another major risk factor is combining cannabis with alcohol which amplifies the impairing effects of both substances.”

With alcohol, there are set limits on when a person can drive – the federal limit to legally drive in the United States is ablood alcohol content of 0.08%. If your blood alcohol level is above this level, you are presumed intoxicated and subject to a DUI (driving under the influence) charge. Most states, however, also have “zero tolerance” levels — often .02% or less — that apply to certain groups, such as bus and truck drivers and teenagers (who aren’t supposed to be drinking anyway).

That helpful guidance isn’t available for cannabis — it’s illegal to use marijuana at any level and drive in the United States, just as it is for opioids, methamphetamines or any potentially impairing drug, even if prescribed, according to the National Highway and Transportation Safety Administration.

“There is no hard and fast rule about when it is safe to drive after using cannabis as risk of impairment depends on multiple factors,” Myran said “No one should be driving if they are experiencing any acute psychoactive effects. Canada’s lower-risk cannabis guidelines recommend not driving for at least 6 hours after using cannabis and avoiding cannabis and alcohol together. Waiting longer is safer.”

Much more is needed to curb the growing concern over driving while under the influence of marijuana, experts say.

“Education, education, education. The general public should be educated on the risks associated with cannabis, in particular in the younger strata of the population,” Solmi said. “Also, THC-containing products should contain explicit evidence-based warnings and visuals, exactly as it is done for tobacco cigarettes.”

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