Drivers whose blood carries trace amounts of a metabolite of marijuana can not be found guilty of driving while intoxicated unless there’s actual evidence that the drug impaired them, the Arizona Supreme Court found on Tuesday.

A trial judge dismissed the charges against the man, who in a pretrial motion argued that the blood test did not reveal the presence of THC, or “its metabolite,” Hydroxy-THC. The state, which according to the opinion presented expert witness testimony that it did not test for that because it doesn’t “exist in the blood for very long” and converts quickly to Carboxy-THC, filed an appeal. According to Capital Media services, Carboxy-THC can remain in someone’s bloodstream for up to 30 days.

The opinion provides protection for Arizona’s more than 40,000 medical marijuana users and for drivers visiting from Washington or Colorado, where recreational marijuana use is legal, the article states.

Arizona legalized medical marijuana in 2010. The Arizona Supreme Court opinion does not mention whether the defendant had a prescription for medical marijuana, but does note that the state’s position could be problematic for residents who use the drug legally.

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