New research from the University of Sydney indicates that blood and oral fluid THC concentrations are relatively poor or inconsistent indicators of cannabis-induced impairment.

While there have been efforts in recent years to find something similar to a breathalyzer test to negate fears about cannabis users behind the wheel, the study’s findings suggest that may not be the most effective approach after all. 

Direct comparisons between weed and alcohol when it comes to driving aren’t exactly accurate. Even the feds have noted cannabis users are more adept behind the wheel because they have a situational awareness that is more tuned in than someone who’s drunk. 

“Higher blood THC concentrations were only weakly associated with increased impairment in occasional cannabis users while no significant relationship was detected in regular cannabis users,” said the study’s lead author, Dr. Danielle McCartney of Lambert Initiative for Cannabinoid Therapeutics. “This suggests that blood and oral fluid THC concentrations are relatively poor indicators of cannabis-THC-induced impairment.”

So how did they come to this conclusion? Well, as America is all too familiar, researching cannabis can be extremely difficult, and Australia has often followed the U.S. lead as it pushed cannabis prohibition on the world in the 1900s. So these researchers pooled data from 28 other published studies that involved the test subjects eating or smoking cannabis. From there, they figured out THC concentrations in blood and saliva impact performance on skills associated with driving including reaction time or divided attention.

The first takeaway from the authors was infrequent cannabis users did see some significant correlations between how much weed was in their system and how impaired they were observed to be. But the researchers also wanted to make clear that most of these relationships were “weak” in strength. 

For people who use marijuana on a weekly basis, there was found to be no correlation between THC levels in the bloodstream and impairment. 

“Of course, this does not suggest there is no relationship between THC intoxication and driving impairment,” Dr. McCartney said. “It is showing us that using THC concentration in blood and saliva are inconsistent markers for such intoxication.”

As the authors note, the data they’ve found essentially questions the entire global strategy for establishing how impaired someone is by the amount of marijuana in their bloodstream. 

McCartney noted the results indicate that someone could mistakenly be identified as cannabis-intoxicated when THC limits are imposed by the law. “Likewise, drivers who are impaired immediately following cannabis use may not register as such,” she said.

One of the other surprising things researchers found was people’s own perception about how high they were. The researchers found self-recognition of how high you are was only weakly associated with impairment. The authors argued that this means you should not solely rely on your own perception of whether you’re too high to drive. 

Individuals are better to wait a minimum length of time, between three and 10 hours, depending on the dose and route of administration, following cannabis use before performing safety-sensitive tasks,” said co-author Dr. Thomas Arkel. “Smartphone apps that may help people assess their impairment before driving are currently under development and may also prove useful.”


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