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.

INCONSISTENT STATE LAWS

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 non-psychoactive marijuana compounds for at least some conditions, such as epilepsy.

Some states require 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.

DRIVER SMOKED THE NIGHT BEFORE

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.