Arizona Supreme Court Makes It Harder to Convict Unimpaired Pot Smokers of Driving Under the Influence


Yesterday the Arizona Supreme Court ruled that people whose blood contains a secondary, inactive byproduct of marijuana can drive without violating state law. That means cannabis consumers are less likely to be charged with driving under the influence of a drug (DUID) when they are not actually impaired.

Arizona, one of seven states with a "zero tolerance" DUID law, forbids driving by anyone whose body contains any amount of an illegal drug "or its metabolite." The question for the Arizona Supreme Court was whether "its metabolite" includes carboxy-THC, which is not psychoactive and can be detected up to a month after consumption of cannabis. The court deemed the phrase ambiguous, since it could be understood as referring only to hydroxy-THC, a primary metabolite that is psychoactive. In light of the legislature's aim to prevent impaired driving, the court said, that interpretation makes more sense, especially since the alternative, including any chemical trace left by the metabolism of an illegal drug, would lead to "absurd results":

This interpretation would create criminal liability regardless of how long the metabolite remains in the driver's system or whether it has any impairing effect. For example, at oral argument the State acknowledged that, under its reading of the statute, if a metabolite could be detected five years after ingesting a proscribed drug, a driver who tested positive for trace elements of a non-impairing substance could be prosecuted.

The broader interpretation would therefore "criminalize otherwise legal conduct": medical use of cannabis permitted by state law. "Because Carboxy-THC can remain in the body for as many as twenty-eight to thirty days after ingestion," the court noted, "the State's position suggests that a medical-marijuana user could face prosecution for driving any time nearly a month after they had legally ingested marijuana."

The court noted that a broad reading of "its metabolite" could have absurd results when applied to other drugs as well:

This interpretation would allow the prosecution of an individual who drives after ingesting a legal substance that shares a non-impairing metabolite with a proscribed substance. For example, serotonin, a legal substance, and the proscribed drug bufotenine [a psychedelic found in fly agaric mushrooms and the skin of certain toads] share a common metabolite, 5-hydroxindoleactic acid ("5-HIAA")….Under the State's interpretation of "metabolite," it could prosecute a driver who had 5-HIAA in his or her system after ingesting a legal serotonin supplement or, for that matter, whose blood contains 5-HIAA as a byproduct of naturally produced serotonin.

This decision is reminiscent of a 2010 ruling in which the Michigan Supreme Court narrowly interpreted that state's zero-tolerance DUID law, which says a person may not drive "if the person has in his or her body any amount of a controlled substance listed in schedule 1," which includes marijuana, THC, and "their derivatives." The Michigan Supreme Court said carboxy-THC does not count as a "derivative."

Despite these rulings, it is still possible for unimpaired marijuana consumers to be convicted of DUID in Arizona and Michigan, since any amount of active THC in the blood will suffice. In fact, such convictions are possible even under less strict per se standards, such as Washington's new rule equating five nanograms of THC per milliliter of blood with impairment, which makes it illegal for many medical users to drive even when they are perfectly capable of doing so safely. Given variation in the way people respond to psychoactive substances (including alcohol) , per se standards are bound to unfairly tar some people as public menaces That is a good reason to require additional proof of impairment—unless you are using road safety as an excuse to punish people for consuming politically incorrect drugs, in which case per se (and ideally zero tolerance) is the way to go.

[Thanks to Marc Sandhaus and Richard Cowan for the tip.]