Analysis: THC Levels Not Indicative of Driving Impairment – Norml

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

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