PHOENIX -- Authorities can't prosecute Arizona motorists for driving under the influence of marijuana unless the person is impaired at the time of the stop, the state Supreme Court ruled Tuesday in the latest opinion on an issue that several states have grappled with across the nation.
The ruling overturned a state Court of Appeals decision last year that upheld the right of authorities to prosecute pot smokers for DUI even when there is no evidence of impairment.
The opinion focuses on two chemical compounds in marijuana that show up in blood and urine tests, one that causes impairment and one that doesn't but stays in a pot user's system for weeks.
Some prosecutors had warned that anyone in Arizona who used medical marijuana simply shouldn't drive or they would risk facing DUI charges, a contention that drew the ire of pot advocates who claimed this interpretation of the law criminalized their legal use of the drug after voters approved it in 2010.
Tuesday's state Supreme Court opinion removed that threat in explaining that while state statute makes it illegal for a driver to be impaired by marijuana, the presence of a non-psychoactive compound does not constitute impairment under the law.
Twenty-one states and the District of Columbia allow medical marijuana use, while two states, Washington and Colorado, have legalized the drug for recreational use by adults over 21. Five other states this year adopted laws that allow the use of nonpsychoactive marijuana compounds for at least some conditions, such as epilepsy.
Some states required signs of impairment before someone can be charged with driving under the influence of marijuana. Others have zero tolerance for the presence of any marijuana in the blood, whether in the form of active compounds that cause impairment or inactive compounds that don't, while a few states have limits for how much active marijuana can be in the system, designed to be comparable to the .08-limit for drunken driving.
However, only eight of those states have laws that allowed a driver to be charged with being under the influence for having even marijuana compounds in their systems that don't cause impairment, according to the Marijuana Policy Project.
Last year, the Michigan Supreme Court ruled that medical marijuana users should have some protections and that police must show that a driver is actually "under the influence" of the drug -- meaning impaired -- to seek criminal charges.
Tuesday's ruling arises from the case of an Arizona man who was stopped by police for speeding and later acknowledged having smoked marijuana the night before. Blood tests revealed marijuana compounds in his system, however, not the form that causes impairment, according to court records.
He was charged with driving under the influence of a drug and operating a vehicle with the presence of the drug's metabolite in his system.
The state Supreme Court noted that the language of Arizona's statute is ambiguous and does not make a distinction between the marijuana metabolite that causes impairment and the one that does not when determining whether criminal charges are warranted. Prosecutors had argued that the statute's reference to "its metabolite" when referring to drug compounds detected in a driver's system covers all compounds related to drugs, not just those that cause impairment.
This interpretation "leads to absurd results," the high court panel wrote. "Most notably, 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."
In a dissenting opinion, Justice Ann A. Scott Timmer wrote that the law helps "enhance detection and prosecution of drugged driving" and should remain unchanged. She suggested any constitutional challenges would be better addressed on a case-by-case basis.
Maricopa County Attorney Bill Montgomery expressed disappointment with the ruling, noting the court should have left such a decision up to the Legislature to clarify.
Attorney Michael Alarid III, who represented the man charged in the case, however, said, "we're very pleased, and we're very relieved that it's finally over.
"This does have far-reaching impacts on medical marijuana patients," he added, "and it basically corrects an error in the interpretation of the law."