Analysis: THC Levels Not Indicative of Driving Impairment

New Haven, CT: The presence of THC concentrations in either blood or saliva is an unreliable predictor of impaired driving performance, according to a literature review published in the journal Frontiers in Psychiatry.

Researchers affiliated with Yale University assessed multiple papers specific to the issue of marijuana and driving performance. Consistent with prior reviews, authors reported that the presence of THC in bodily fluids is not a consistent predictor of impairment and that state-imposed per se limits for THC are not evidence-based. 

Authors reported, “While legislators may wish for data showing straightforward relationships between blood THC levels and driving impairment that parallel those of alcohol, the widely different pharmacokinetic properties of the two substances … make this goal unrealistic.”

They added: “[S]tudies suggest that efforts to establish per se limits for cannabis-impaired drivers based on blood THC values are still premature at this time. Considerably more evidence is needed before we can have an equivalent ‘BAC for THC.’ The particular pharmacokinetics of cannabis and its variable impairing effects on driving ability currently seem to argue that defining a standardized per se limit for THC will be a very difficult goal to achieve.”

Researchers concluded: “Until there is more evidence-based consensus of opinion on meaningful thresholds for per se laws, we would recommend against reliance on such legislation. This is particularly the case given the significant inconsistencies in threshold values currently determined by different states in the US, and the rather weak scientific basis for such decisions. Any such laws cannot claim to be strongly based on current scientific evidence, which suggest collectively that standard based on detectable blood THC levels are not useful.”

Their findings are consistent with those of numerous other studies and expert review panels concluding that the presence of THC is an unreliable indicator 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. 

The study’s authors acknowledged that acute cannabis-induced intoxication can influence driving behavior, but also recognized that “the relative risk of such impaired driving is significantly lower than other legislated drug use while driving, such as that resulting from alcohol.”

Full text of the study, “Cannabis and Driving,” appears in Frontiers in Psychiatry. Additional information regarding cannabis and psychomotor performance is available from NORML.