Analysis: THC Blood Concentrations Are Not Correlated With Impairment
How much THC in a person’s system is too much when it comes to safely operating a motor vehicle? Due to the complexities of human biology, that question is far more difficult to answer than many policymakers would lead the public to believe.
An infrequent user can consume very little cannabis and be significantly impaired, and a frequent consumer can refrain from consumption for multiple days and yet still have a considerable amount of metabolized THC in their system. There is no ‘one size fits all’ THC threshold that automatically equates to impairment.
That fact is demonstrated in a recent analysis conducted by researchers in Norway which found that THC blood concentrations are not correlated with behavioral impairment. Below is more information about the analysis and its findings via a news release from NORML:
Oslo, Norway: The presence of THC in blood is poorly correlated with behavioral impairment, according to data published in the Forensic Science International.
Norwegian researchers assessed the relationship between drug concentrations and impaired psychomotor performance in a cohort of over 15,000 individuals suspected of drugged driving and 3,684 drug-free controls. Driving performance was assessed by participants’ performance on a clinical test of impairment (CTI).
Consistent with prior literature, “The correlation between drug concentration was high for ethanol, … but low for THC.”
Specifically, authors determined, “For THC, the median drug concentrations changed little between drivers assessed as not impaired and impaired.”
They concluded, “The lack of a close relationship between drug concentration of THC and degree of impairment at the individual level is in accordance with several observations from experimental studies [where participants engaged in the] controlled intake of cannabis.”
The authors’ findings are consistent with those of numerous studies reporting that neither the detection of THC nor its metabolites in blood 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, “The relationship between clinical impairment and blood drug concentrations: Comparison between the most prevalent traffic relevant drug groups,” appears in Forensic Science International. Additional information is available from the NORML Fact Sheet, ‘Marijuana and Psychomotor Performance.’