In states where some form of marijuana is already legal, one problem becomes glaringly evident: It’s really, really hard to tell just how high you are. 

This fall, as many as 11 states are putting the legalization of marijuana (recreational, medicinal, or both) on their ballots. When it comes to a Marijuana DUI, this can be very tricky. Though industry leaders are working to define how we measure such things, there is a whole lot of gray area when it comes to impairment. 

And this could mean big trouble for drivers, as police and scientists struggle to define the "acceptable" amount of THC to determine impairment. 

Current Marijuana DUI laws fall short 

Currently, only 6 of the 26 places where marijuana is allowed have set limits for the amount of THC you can have in your system: 

  • Pennsylvania 
  • Ohio 
  • Colorado 
  • Montana 
  • Nevada 
  • Washington State       

And even in states that do have a threshold, it is generally an arbitrary number, with little or no correlation to actual impairment. 


Marijuana DUI
So what happens if you’re using marijuana in a legal manner, but you don’t live in one of those states? That can spell big trouble for drivers, because it is up to the defendant to prove that he or she is not impaired. In fact, nine states (including Arizona) have laws that state any THC in your system means you are impaired, which makes, according to Mark Kleinman, a professor at NYU who specializes in Criminal Policy, absolutely no sense. THC metabolites can linger in the system for weeks.  "A law against driving with THC in your bloodstream is not a law you can know you are obeying except by never smoking marijuana or never driving," Kleinman said. 

How do they test for THC? 

A blood test is used to detect marijuana in your system, but the tricky part is that some drugs stay in your system long after you cease to feel the effects. This makes it difficult to enforce a marijuana DUI. Marijuana sticks around the longest, according to research. It has been detected several months after use. Generally, the effects wear off after a few hours. So even with THC in your system, it could be from weeks or months ago. 

Marijuana DUI TestThe problem is, each person metabolizes it in a different way, and there are so many different varieties a test may be needed for each subtype. Jeremy Plumb, a chemist who is also the owner of Newcleus Nursery, explains the difficulty: "Cannabis is the most phytochemically complex plant on the planet." 

Read more about the difficulties of measuring the amount of marijuana in your system.

So how bad is driving under the influence of marijuana? 

Besides, Kleinman argues, driving while under the influence of marijuana isn’t really all that dangerous. It doubles the risk of a crash, which sounds bad, but using a hands-free telephone (legal in all 50 states, by the way) quadruples your risk. Marijuana use is comparable to, according to Kleinman, "driving with a noisy child in the back of the car." 

What this means for drivers in Arizona 

Arizona seems particularly unfriendly to people who have permission to use medicinal marijuana. In November, the Arizona Supreme Court ruled that even if you have a medical marijuana card, you are not exempt from the no-trace policy. And though Chief Justice Scott Bales said that "The patient may establish an affirmative defense to such a charge by showing that his or her use was authorized by the AMMA", the burden of proof is still on the defendant. And as I wrote about earlier in this blog, that proof can be very hard to find. 

So drivers in Arizona and other no-tolerance states face a very difficult fight if they are accused of a marijuana DUI. 

Medical Marjuana DUILike any type of DUI, there is certain protocol you need to follow if you are accused of driving while impaired. Be courteous to the arresting officer, but don’t talk about whether or not you have been using (legally or illegally) marijuana. In fact, don’t talk at all. Whatever you say WILL be used against you. You also, have a right to refuse the roadside tests. And ask to talk to your lawyer before giving a blood or breath sample. Make sure you have a good lawyer on your side, who knows the law and is willing to stand up for your rights.

Contact Burges McCowan, Phoenix Criminal Defense Attorney, for a free consultation.