Study: THC Blood Levels Do Not Correlate With Impairment

In a recent study published in the Journal of the American Medical Association (JAMA) Network Open, researchers have taken a closer look at the effects of cannabis on driving abilities, particularly focusing on older adults aged 65 to 79 years. The study, led by Patricia Di Ciano, PhD, alongside a comprehensive team of experts, delves into the nuanced relationship between the consumption of cannabis, its impact on driving performance, and the amount of THC found in participants’ blood.

In summary, the authors noted that although the evidence suggested cannabis use could impact driving performance, “blood THC level was not correlated with driving behavior.”

The study’s participants, who were regular users of high-potency THC-dominant cannabis, demonstrated an increase in weaving and a decrease in speed 30 minutes after consumption. However, these changes in driving performance were not directly correlated with blood THC concentrations. Impairment seemed to fade within a few hours of consumption, with the authors noting “the associations between cannabis and driving were apparent at 30 minutes but not 180 minutes after smoking.” Interestingly, self-reports of impaired driving persisted for 3 hours, indicating a heightened awareness among participants of their potential impairment.

In response to the study, NORML released a statement calling for more sensible regulations around cannabis impairment.

“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.”

– Excerpt from NORML’s statement

DRUID is a mobile app produced by Impairment Science, Inc. that uses “neuroscience to assess a user’s level of cognitive and motor impairment,” according to their website. The app can be used by individuals to determine their own impairment, as well as used for numerous business applications. It was implemented in 2021 by the Department of Public Safety in Highland Park, Texas, to track officers’ fatigue levels and help determine optimal shift schedules.

Considering that THC blood tests are currently used by most law enforcement agencies to determine if drivers are under the influence, the lack of a clear correlation between this variable and driving performance challenges the effectiveness of these regulations and highlights the need for a more nuanced approach to determining impairment.