Marijuana use—both recreational and medical—has become more socially acceptable over the years. As a result, the number of states that have legalized marijuana has grown. But the drug remains illegal in many states—and is still illegal under federal law.
Just because marijuana is legal in your state doesn't mean it's legal to drive while impaired by it. In fact, driving under the influence of marijuana is illegal in every state.
Many states—including California, Nevada, Oregon, Massachusetts, Maine, Washington, Colorado, Illinois, Michigan, Vermont, Alaska, and Washington D.C.—have legalized at least some recreational marijuana use. And a handful of states have decriminalized recreational marijuana use. Most states also have legalized marijuana for medical use.
So, how does the legalization or decriminalization of weed affect DUI laws? Generally, it has no effect. After all, alcohol and prescription drugs are legal, but you'll get a DUI if you drive while impaired by them. For these purposes, marijuana is treated similarly to other legal substances.
Most states that have decriminalized marijuana haven't changed their DUI laws. The laws of these states already covered driving while impaired by marijuana (and alcohol and other drugs) and continue to do so.
All states have two types of DUI charges: "per se" and "impairment." The difference between the two charges is how the prosecution proves the driver was "under the influence."
In per se DUI cases involving alcohol, the prosecution just needs to show that the driver had a blood alcohol concentration (BAC) of .08% or more (.05% or more in Utah.) The thinking is that anyone who has a BAC of at least .08% is under the influence and shouldn't be driving.
Most per se laws refer only to alcohol—they don't prohibit driving with a specific amount of drugs in your system. So, a motorist who is under the influence of only marijuana typically can't be convicted of a per se DUI.
But a handful of states have adapted the per-se-DUI concept to drugged driving. In these states, motorists can be convicted of a per se drug DUI for driving with a certain concentration of drugs in their system.
Several states that legalized recreational marijuana use have a "per se marijuana" DUI charge. For example, Washington's DUI law has a provision that prohibits driving with a concentration of five nanograms or more of THC (the primary psychoactive ingredient in marijuana) per milliliter of blood. (Wash. Rev. Code § 46.61.502 (2023).)
To prove a per se marijuana DUI, a prosecutor must give the court chemical test results. The test is conducted after the driver is arrested and typically involves giving a blood or urine sample. The sample is then tested in a lab. For a conviction, the results must show that the motorist's blood had a certain concentration of THC.
Critics of THC testing argue that THC has little to no correlation to impairment. They rely on studies showing that THC can appear in a chemical test days or weeks after a motorist used marijuana. And, although it's unlikely, cannabidiol (CBD) use could create a false positive for marijuana use.
The focus of impairment DUI charges is on how the drugs or alcohol affected the driver—not on the actual amount of a substance in the driver's system.
And, if the driver is impaired, it doesn't matter whether the alcohol or drug use was legal. That's why states haven't had to change their DUI laws in the wake of marijuana decriminalization: A driver who ingests marijuana legally—but is impaired—can still be convicted of an impairment DUI.
All jurisdictions have an impairment DUI charge. But the degree of impairment the prosecution must prove to get a conviction varies by state.
In Nevada, for instance, prosecutors must prove the defendant, as the result of the drugs or alcohol ingested, was impaired to the point that they were "incapable of safely driving or exercising actual physical control of a vehicle." (Nev. Rev. Stat. § 484C.105, .110 (2023).)
And under Massachusetts law, a person is "under the influence" if the substance "diminished the [person's] ability to operate a motor vehicle safely." (Commonwealth v. Connolly, 394 Mass. 169 (1985).)
The consequences of a DUI conviction are serious, and DUI law varies by state. If you've been arrested for or charged with driving under the influence, get in touch with an experienced DUI lawyer. A qualified DUI attorney can explain how the law applies to the facts of your case and help you decide on the best course of action.