The number of states legalizing medical marijuana continues to climb, now nearing two dozen. Washington state and Colorado have already legalized recreational use, with Oregon, Alaska and Washington DC about to vote on the issue this coming election cycle. With all these states and localities changing their approach to marijuana, police departments have to decide how these changes will apply to laws governing driving under the influence (DUI).

Arizona is among the states which have legalized marijuana use for medical purposes in recent years. About 50,000 state residents now have medical marijuana licenses since 2010. Now an Arizona Court has ruled that drivers can still be charged with a DUI for having marijuana in their systems, even if they have a valid medical marijuana license.

In December of 2011, Travis Darrah was charged with two counts of DUI, based both on impairment, and on the presence of marijuana in his system. Although a jury acquitted Mr. Darrah of the impairment charge, they did convict him of the second charge because the DUI law banned driving while having traces of the drug in his system.

Mosts states have similar charges available for a DUI related to alcohol. A driver can be charged for a DUI based on impaired driving, or based on the level of alcohol in the blood. Nationwide, the DUI baseline for impaired driving is 0.08 percent blood alcohol content (BAC).

Darrah appealed the DUI conviction. He argued that medical marijuana users are immune from prosecution under the Arizona Medical Marijuana Act based solely on the presence of marijuana in the body, unless they are impaired. However, a state panel of three judges disagreed, and upheld his conviction. Judge Michael Brown wrote that if voters had wanted to protect medical marijuana users from prosecution of DUI, the law would have contained such language.

The state of Washington and Colorado both have set legal limits for the concentration of THC in the blood that is sufficient to constitute a DUI. Arizona, and other states, have not set a limit for marijuana concentration, making any level of THC in the blood enough to be considered a DUI. According to an article in the National Center for Biotechnology Information, THC can remain in the system days after use.

As more states confront the changing attitude toward medical marijuana use, and even recreational use, they will be forced to address how this affects law enforcements approach to marijuana DUIs. A Washington Sheriff is even voicing his support for legalized recreational marijuana for Oregon's upcoming election.

King County Sheriff John Urquhart can be seen in an Oregon state campaign ad implying legalized marijuana has led to a decrease in Washington’s DUIs. The Oregon State Sheriff's Association has indicated those claims are misleading. According to local news fact-checking, DUIs in Washington have decreased since marijuana was legalized in December 2012, but the decrease in DUIs has been a continuing trend since at least 2009. In contrast, marijuana DUIs have increased since legalization, from about 988 in 2012 to 1,362 in 2013.