Weed smokers and placebo takers show same driving performance after four hours

“The complete lack of correlation between blood concentrations and driving performance was somewhat surprising. It’s strong evidence against developing per se driving under the influence statutes."

Angela Stelmakowich 5 minute read January 28, 2022

Simulator measured common driving variables, such as swerving in the lane, responding to divided attention tasks and following a lead car. / UC San Diego Health Sciences

U.S. researchers exploring simulated driving performance among regular cannabis smokers saw reduced skills behind the wheel, but “indistinguishable” performance at 4.5 hours compared to those taking a placebo.

Acknowledging that regular users who smoked when and how much they felt like “resulted in simulated driving decrements. However, when experienced users control their own intake, driving impairment cannot be inferred based on the THC content of the cigarette, behavioural tolerance or THC blood concentrations,” authors write in the study, published this week in JAMA Psychiatry.

Even so, investigators from multiple U.S. caution that users may perceive that their impairment has ended before it has. Subjects were increasingly willing to drive at about the 90-minute mark, accounting for 68.6 per cent of participants, despite their driving performance not improving.

This “may indicate a false sense of driving safety. Worse driving performance is evident for several hours post-smoking in many users, but appears to resolve by four hours 30 minutes in most individuals,” researchers report. “Performance was indistinguishable from the placebo group at 4.5 hours.”

The finding appears to be in line with an Australian review released last year.

Investigators with the University of Sydney’s Lambert Initiative for Cannabinoid Therapeutics suggested at the time that they had identified a range of times during which a person is impaired after inhaling or orally ingesting weed.

Australian researchers recently identified a window of impairment

The “window of impairment” was three to 10 hours for “moderate to high doses” of THC, they wrote in the review published in the peer-reviewed journal, Neuroscience & Biobehavioral Reviews.

Earlier research from the group, this time released in December of 2020, indicated that CBD does not impair driving and that moderate amounts of THC produce mild driving impairment lasting up to four hours.

In the most recent clinical trial, subjects smoked a placebo, a joint with 5.9 per cent THC or one with 13.4 per cent THC. Conducted from February 2017 to June 2019 at the Center for Medicinal Cannabis Research, University of California San Diego (UCSD), authors did their analysis from April 2020 to September 2021.

Drivers were tested on driving simulation tests over several hours, notes a statement from the UCSD.

Driving skills assessed included swerving and following a lead car

With regard to the Composite Drive Score — which assesses key simulated driving variables like swerving in the lane, responding to divided attention tasks and following a lead car — the THC group significantly declined at 30 minutes and at 90 minutes post-smoking compared to the placebo group. There were borderline differences at 3 hours, 30 minutes, but not at 4 hours, 30 minutes.

The drive score did not differ based on THC content or use intensity, meaning quantity times frequency, in the past six months “despite post-smoking blood THC concentrations being higher in those with the highest use intensity.”

The Composite Drive Score assessed key simulated driving variables such as swerving in the lane, responding to divided attention tasks and following a lead car. / Azure-Dragon / Getty Images/iStockphoto

“The complete lack of correlation between blood concentrations and driving performance was somewhat surprising. It’s strong evidence against developing per se driving under the influence statutes,” suggests study co-author Robert Fitzgerald, Ph.D., professor of clinical pathology at UCSD’s School of Medicine.

“Per se laws provide a legal shortcut, essentially eliminating the requirement to prove the driver’s ability was impaired,” according to the Canadian Centre on Substance Use and Addiction.

Cannabis should not be treated the same as alcohol

“This groundbreaking research indicates that cannabis use does impair driving ability, but factors differ from alcohol,” California State Assembly member Tom Lackey says in the statement. “For example, these data show that per se laws for THC levels are not supported scientifically.”

Despite diminished driving skills in cannabis users compared to the placebo group, in general, that was not the case for all weed smokers. Researchers report that about 50 per cent of the smokers “could be described as impaired,” the university reports.

Additionally, subjects administered both levels of THC “performed similarly, suggesting that users ‘self-titrated’ by smoking in such a way to achieve similar highness levels,” according to the university statement.

There is an urgent need to understand what impacts impairment

Study authors emphasize that expanding cannabis medicalization and legalization highlights “the urgency to understand the factors associated with acute driving impairment.”

That being the case, “further research is needed on the impact of individual biologic differences, cannabis use history and administration methods on driving performance,” they write.

Newly released research looking at the effect of both cannabis and alcohol use on driving determined that the combination “is more detrimental to driving performance than either used in isolation,” according to the Society for the Study of Addiction.

The meta-analysis of 57 studies found the effects of cannabis on driving performance were similar to those of low blood alcohol concentrations.

“Cannabis use on its own reduces lateral control of the vehicle (the ability to stay in the lane) even though drivers under the influence of cannabis slow their driving speed, possibly in an attempt to compensate for their impaired state,” the society statement reports.

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