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THC Levels Not Correlated With Changes In Driving Performance


Public policies related to driving under the influence as they pertain to cannabis is an extremely important topic. All cannabis consumers should want to keep roadways safe and treat driving while intoxicated by any substance as a very serious matter.

With that being said, just because someone consumes cannabis does not automatically mean that they are too impaired to drive. Also, just because someone has cannabis in their system it doesn’t mean that they are impaired, and for that matter, it doesn’t mean that the cannabis in the person’s system indicates that they consumed recently.

Cannabis can stay in a person’s system for a very long time, and due to how fast the body metabolizes cannabis, having cannabinoids in the system could mean that the person consumed cannabis just hours prior, or a month prior – there’s no scientific way to know for sure once cannabinoids have been metabolized.

Researchers in Canada recently examined THC levels in subjects’ blood and whether or not it correlated to changes in driving performance. Below is more information about the results of the study via a news release from NORML:

Toronto, Canada: The presence of THC in blood is not predictive of detriments in psychomotor performance, according to driving simulator data published in the journal JAMA Network Open.

Researchers affiliated with the University of Toronto assessed subjects’ simulated driving performance at baseline and then again 30 and 180 minutes after smoking cannabis. Participants were between the ages of 65 and 79 years old and smoked cannabis (mean THC potency: 19 percent) ad libitum prior to driving.

Subjects exhibited “small changes in SDLP [weaving]” 30 minutes after cannabis inhalation. Investigators described these changes as less pronounced than those associated with drivers with a BAC below 0.05 percent.

Consistent with the results of prior studies, participants decreased their speed after smoking and were more likely to self-assess their performance as “impaired.” Cannabis use did not impact participants’ reaction times.

Subjects’ simulated driving performance returned to baseline within three hours.

The study’s authors concluded: “The purpose of the present study was to investigate the association between cannabis and driving and blood THC levels in older adults. … There was no correlation between blood THC concentration and SDLP [standard deviation in lateral positioning] or MS [mean speed]. … The lack of correlation between driving and blood THC fits within emerging evidence that there is not a linear relationship between the two.”

That conclusion is consistent with numerous studies reporting that neither the detection of THC nor its metabolites in blood and/or other bodily fluids is predictive of impaired driving performance. As a result, NORML has long opposed the imposition of per se THC limits for motorists and has alternatively called for the expanded use of mobile performance technology like DRUID.

Full text of the study, “Cannabis and driving in older adults,” appears in JAMA Network Open. Additional information is available from the NORML Fact Sheet, ‘Marijuana and Psychomotor Performance.’