Kudos to the editors for acknowledging that proposed per se thresholds for marijuana are not evidence-based and may result in inadvertently prosecuting non-impaired drivers (“Marijuana OUI not as simple as drawing blood,” Dec. 21).

This is because both THC and its byproducts are detectable for periods of time extending well beyond any reasonable period of impairment. According to data published this spring in the scientific journal Forensic Science International, residual levels of THC may be present in subjects for up to seven days after they have ceased their use of cannabis.

As acknowledged by no less than the U.S. National Highway Transportation Safety Administration, “It is difficult to establish a relationship between a person’s THC blood or plasma concentration and performance impairing effects. … It is inadvisable to try and predict effects based on blood THC concentrations alone.”

Testifying before Congress last year, Dr. Jeffrey Michael, associate administrator for Research and Program Development at NHTSA, affirmed: “The available evidence does not support the development of an impairment threshold for THC (in blood) which would be analogous to that (of) alcohol. … [A] more precise association of various THC levels and degrees of impairment are not yet available.”

As states like Maine consider amending their cannabis consumption laws, lawmakers would be best-served to avoid amending traffic safety laws in a manner that relies solely on the presence of THC or its metabolites as determinants of guilt. Otherwise, the imposition of traffic safety laws inadvertently may 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.

Paul Armentano, deputy director

National Organization for the Reform of Marijuana Laws (NORML)

Washington, D.C.

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