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thc-impaired driving

New Research on THC-Impaired Driving Shows Not All Skills Affected Equally by Cannabis

As cannabis is increasingly legalized and decriminalized around the world, concern has mounted about THC-impaired driving. Researchers have not come to a consistent definition for impairment due to cannabis, and we currently do not have means to easily and effectively measure the degree to which drivers are affected on the road. 

However, new research begins to deepen our understanding of how impairment is related to contextual factors. Researchers at the University of Sydney recently conducted a large-scale meta-analysis to better understand current research on how drivers are influenced by weed. The authors compiled data from 80 scientific studies published in the last 10 years investigating how THC affects driving performance and driving-related cognitive skills. 

Using statistical techniques on the qualifying studies, the authors were able to characterize the relationship between impairment and specific factors like type and complexity of skill, cannabis behavior and frequency of use, dose of THC, route of administration, and time until normal cognitive functioning recovered. 

The resulting meta-analysis supports the findings of previous research showing that THC administration impedes driving performance. However, with much more data and statistical power in their study design, the researchers were able for the first time to tease out a much more nuanced and granular understanding of how THC affects drivers, and for how long.

THC-Impaired Driving: Not All Highs are Created Equal

The study suggests that more demanding or complex skills were more impaired by THC. Several measures of driving performance—lateral control, standard deviation of lane position (i.e., the ability to stay in one’s lane without weaving), and reaction time—significantly deteriorated after THC administration. Other driving-related cognitive skills such as abstract reasoning, divided attention, and information processing, were also impaired after consuming THC. 

However, it’s noteworthy that other skills were not affected by THC, including car following, speed and speed variability, sensory discrimination, and time perception. 

Furthermore, just as other substances like alcohol affect and impair people differently due to a range of factors (e.g., frequency of alcohol consumption, body size, gender, time of last meal, to name a few), THC does not affect cannabis users uniformly. 

“We found that ‘regular’ cannabis users; that is, individuals who used cannabis weekly or more often, experienced less THC-induced impairment than ‘occasional’ cannabis users,” says Danielle McCartney, the lead author of the paper and a researcher at the University of Sydney’s Lambert Initiative for Cannabinoid Therapeutics.

Why do we observe this effect? There have been a number of pharmacodynamic mechanisms (the way cannabinoids interact with the body) proposed as possible explanations. One possibility is the downregulation of cannabinoid receptors, meaning that over time after frequent cannabis use, the number of receptors on the surface of target cells may decrease, leading to reduced sensitivity to cannabinoids. (CB1 receptors are primarily located on nerve cells while CB2 receptors are largely found on the surface of white blood cells and other immune-system cells).

Other possibilities that could explain these differences are receptor internalization (where cannabinoid receptors are taken inside the cell’s membrane so they are less available to bind to cannabinoids on the surface of the cell) and receptor conformational change (where a change is induced in the shape of the receptor, in turn changing its function and/or ability to bind to cannabinoids). 

THC-impaired driving
The review found that THC-impaired driving affects different consumers in different ways, and that not all skills are affected equally by impairment.

Route of Administration Plays a Part

The administration of cannabis (the way THC enters the body), makes a significant difference in how users are affected, says Danielle McCartney: “Orally ingested THC (in an oil, capsule, or spray) and inhaled THC (smoked or vaporized) have different pharmacokinetic profiles.” 

Inhalation results in a sudden high peak in blood THC concentration while oral ingestion produces a lower peak concentration over a longer period of time. “This means that impairment peaks at different times,” says McCartney, and the amount of time it takes for users to recover and regain normal functioning will be different depending on how they consume THC. 

Recovery Time Varies

The amount of time it takes for cannabis users to fully recover has not been well defined in the literature. McCartney and her colleagues, however, built a model which predicted that most driving-related skills recover within about five hours, and almost all skills recover within seven hours of inhaling 20 milligrams of THC. 

The recovery time for inhaling 10 milligrams of THC is shorter, and skills are predicted to return to normal within three to five hours. Recovery after oral ingestion takes longer, however, because THC concentration in the blood peaks later. 

Of course, this research gives users a better sense of when it’s safe to drive after consuming THC, providing evidence-informed guidelines to users. But it also highlights the lack of evidence-informed policy with respect to THC-impaired driving.

Says McCartney, “The mismatch between the estimated length of impairment (3 to 10 hours) and the length of time THC can be detected in biological matrices (up to 7 days in blood and longer in extreme cases) would indicate that biomarker-based methods of identifying cannabis intoxicated drivers are ineffective.” Detecting THC in a user’s system long after driving skills have recovered doesn’t provide useful information about who should or should not be on the road. “Our legal frameworks need to focus on the period of time when users are most at risk to themselves and others,” McCartney says.

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The Future of Policy on THC-Impaired Driving

While this research provides valuable information and guidance to recreational users, it tells us much less about the effects of driving for medical marijuana patients. “The majority of studies completed to date have involved young, healthy participants,” says McCartney, but she hypothesizes that “When used to alleviate a medical condition, under careful supervision, THC may have minimal effects on driving.”

In fact, some preliminary research suggests that medical patients may be less impaired on the road than recreational users, especially once their cannabis regimen has stabilized. However, this population is extremely under-studied and more investigation is needed so that we can incorporate evidence into on-the-road policies. 

Of course, the paucity of high-quality research is due in large part to the impediments to funding and conducting it; more and better research on THC-impaired driving will continue to be obstructed until cannabis is no longer classified as a schedule I substance.