A recent meta-study conducted by researchers from the University of Sydney’s Lambert Initiative has shed light on the inadequacy of using blood or saliva THC concentrations as reliable indicators of cannabis-induced driving impairment. These findings highlight the need for more nuanced impairment tests instead of relying on the development of roadside marijuana breathalyzers akin to those used for alcohol.
As the global legal recreational use of cannabis continues to rise, reforming cannabis-driving laws becomes increasingly pressing. According to Iain McGregor, academic director of the Lambert Initiative for Cannabinoid Therapeutics at the University of Sydney, there is a clear necessity for more reliable methods of identifying cannabis impairment in both driving and workplace settings.
The concept of a marijuana breathalyzer capable of detecting THC has been discussed for several years, with various devices currently in development to accurately measure THC levels in breath and saliva, correlating them with blood THC levels. The underlying assumption has been that blood THC concentrations directly correlate with driving impairment, much like how blood alcohol levels are associated with impairment. However, this new research challenges that assumption, suggesting that there may not be a consistent link between blood THC concentrations and driving impairment.
The study analyzed data from 28 published studies encompassing 822 driving-related outcomes. The overall finding indicates a lack of consistent association between THC biomarkers and driving impairment. Notably, when distinguishing between regular and occasional cannabis users, the correlation was even weaker, implying that measuring impairment is particularly challenging in frequent marijuana consumers.
Lead author Danielle McCartney highlights that higher blood THC concentrations only demonstrated a weak connection to increased impairment in occasional cannabis users, while no significant relationship was observed in regular cannabis users. This suggests that blood and oral fluid THC concentrations are unreliable indicators of cannabis-THC-induced impairment.
It is important to note that these findings do not negate the existence of a relationship between impaired driving and THC intoxication. Instead, they reveal the inconsistency of THC levels in blood or saliva as reliable indicators of the type of intoxication that could lead to driving impairments.
The implications of this research are particularly significant as many countries have already established legal limits for blood plasma THC concentrations when driving under the influence of cannabis. However, McGregor emphasizes that cannabis intoxication should not be approached in the same manner as alcohol intoxication. Unlike alcohol, where consistent associations exist between blood alcohol levels and driving impairment, the same cannot be assumed for cannabis.
For instance, a regular cannabis user may exhibit no signs of impairment despite registering high THC blood concentrations, while an occasional user could display low THC levels while being completely intoxicated and unfit to drive. THC intoxication involves a more complex metabolic scenario compared to alcohol intoxication, and blood or saliva THC levels may not accurately reflect acute THC levels in the brain.
One intriguing finding from the study was the absence of a consistent link between driving impairment and subjective reports of intoxication. This suggests that individuals under the influence of cannabis may be unable to subjectively assess their own fitness to drive.
Thomas Arkell, a co-author of the study, emphasizes the need for further research into objective measures of impairment. He suggests that smartphone apps currently being developed to measure impairment could aid individuals in better understanding their subjective fitness to drive.
In summary, this meta-study underscores the limitations of using blood or saliva THC concentrations as reliable indicators of cannabis-induced driving impairment. The research calls for the development of more nuanced impairment tests and objective measures to accurately assess fitness to drive. As the understanding of cannabis impairment evolves, advancements in technology, such as smartphone apps, may assist individuals in determining their level of impairment before getting behind the wheel.
The study was published in the journal Neuroscience & Biobehavioral Reviews.