Analysis: THC Per Se Thresholds Are Unreliable Indicators of Psychomotor Impairment

Herborn, Germany: The presence of THC concentrations in blood is an unreliable predictor of impaired driving performance, according to a literature review published in the journal Frontiers in Psychiatry

A team of investigators from Germany and the United States reviewed multiple scientific papers published over the past decade assessing the relationship between THC blood levels, driving performance, and motor vehicle accident risk. 

They concluded, “[T]here is no clear overall relationship with THC blood or serum levels and driving skills or crash risk. … Not surprisingly, there is no unanimous agreement on potential THC legal cut-off levels. … Therefore, the various THC concentrations used to define a cannabis-related driving offense in EU [European Union] countries and some US-states varying between 1 and up to 7 ng/ml alone may not be appropriate to evaluate driving skill impairment comprehensively.”

The findings are consistent with those of numerous other studies and expert review panels concluding that the presence of THC is an unreliable predictor of either recent cannabis exposure or impairment of performance. A 2019 report issued by the Congressional Research Service similarly determined: “Research studies have been unable to consistently correlate levels of marijuana consumption, or THC in a person’s body, and levels of impairment. Thus, some researchers, and the National Highway Traffic Safety Administration, have observed that using a measure of THC as evidence of a driver’s impairment is not supported by scientific evidence to date.”

NORML has long opposed the imposition of THC per se thresholds for cannabinoids in traffic safety legislation, opining: “The sole presence of THC and/or its metabolites in blood, particularly at low levels, is an inconsistent and largely inappropriate indicator of psychomotor impairment in cannabis consuming subjects. … Lawmakers would be advised to consider alternative legislative approaches to address concerns over DUI cannabis behavior that do not rely solely on the presence of THC or its metabolites in blood or urine as determinants of guilt in a court of law. Otherwise, the imposition of traffic safety laws may inadvertently become a criminal mechanism for law enforcement and prosecutors to punish those who have engaged in legally protected behavior and who have not posed any actionable traffic safety threat.”

In recent months, lawmakers in two states – Indiana and Nevada – have rolled back their THC per se laws. 

Full text of the study, “Cannabis use and car crashes: A review,” appears in Frontiers in Psychiatry. Additional information regarding cannabis and psychomotor performance is available from NORML.