When Are You Too High To Drive?

Cannabis legalization is spreading across the United States, both recreationally and medically. This raises thorny legal questions, but let’s just focus on one. When are you too high to drive?

That’s a tougher question than it seems, especially for people who use medicinal cannabis. When both states and the federal government made cannabis illegal, any suggestion of cannabis was a slam-dunk for DUI. Now, it’s much murkier, especially with medical cannabis. The Arizona Court of Appeals ruled late last year that users of medical cannabis can contest their DUIs. Now officers must prove that the driver was too high to drive, rather than just assuming that any amount of cannabis means the driver is too high.

Randall B. Isenberg, a DUI lawyer in Dallas, said, “Finding someone legally DUI on medical marijuana is hard. There is no nationwide standard for marijuana intoxication like there is for alcohol. Also, marijuana can have different effects at the same dosage in different people. That’s why some states have to provide additional proof that medical marijuana drivers were actually intoxicated at the time of an arrest.”

The Troubles with THC Measurement

Let’s break this statement down. First, what are the limits? All states with legalized cannabis in any form state that if an officer determines that your driving was bad enough, that could be evidence of intoxication. But using a measurement of THC in the body, like we do with blood-alcohol content measurements for drunk driving, has scientific problems.

THC byproducts last much longer in the body than alcohol does because THC is fat-soluble. These byproducts are measured with chemical tests. Since they aren’t flushed out through urine, these byproducts can persist long after any intoxication is gone. Therefore, medical cannabis users are in a precarious situation. If you need cannabis to deal with an illness, does that mean you can’t drive for the rest of your life? That’s what the Arizona case was about.

No Hard Limit for Intoxication

Prosecutors would love to have a hard line for intoxication like the 0.08 limit used in most states for alcohol intoxication. Several states have ruled that 5ng/ml of THC metabolites in the body is an adequate limit. However, in chronic users, these metabolites can build up over time and create an impression that someone is very intoxicated when they are perfectly safe to drive.

There is a rush right now to create the equivalent of a THC breathalyzer that directly measures the core psychoactive compound rather than measuring metabolites. There are devices in Europe that can detect active THC with a swab test, but it doesn’t say how much THC may be active. Hound Labs and several other companies are working hard to get a THC breathalyzer to market. Once someone figures out how to measure active THC in the body, rather than metabolites, prosecutors can start getting the data they need to pursue cases with confidence.

Yet even if we had accurate measurements, there is still the question of what the actual limit is before you’re too high to drive. The research isn’t there. Thanks to the decades of prohibition of the drug, including for research, there’s no hard data on how much THC is needed to make someone unsafe to drive. This is the fundamental question that must be answered before we can really write laws that match the realities of the effects of THC on the body.

Until this question is answered and there is an accurate way to measure active THC in the body, consumers of medical cannabis will need to carefully watch their driving or forego it altogether. The risks of getting hit with a DUI are too great.