As more states legalize marijuana (cannabis), readers have asked if there is a legal blood limit considered “driving under the influence” (DUI), as there is for alcohol. The answer is complicated, but in short, unlike with alcohol where there is a national standard for drunk driving—0.08 g/dL (grams per deciliter) blood alcohol concentration, with states having the option to authorize additional statutes—there is no uniform legal limit for marijuana across states.
A few states, however, have set their own legal driving limits for “presumed” impairment for marijuana, based on blood levels of its main psychoactive chemical, tetrahydrocannabinol (THC)—for example, 5 ng/mL (nanograms per milliliter) in Washington, Montana, and Colorado. If the blood level is at or above these limits, the driver can be considered DUI, whether use of the marijuana was for medical or recreational reasons.
In some states (including California, Florida, Oregon, Nevada, Colorado, and Montana), a driver can be arrested for suspicion of DUI based on the totality of circumstantial evidence of impairment—such as the police officer observing erratic driving, smelling marijuana or finding marijuana or paraphernalia in the car, or doing typical field sobriety testing (which may involve asking the driver to balance on one foot or walk a straight line). In California, Colorado, and some other states, police officers are specially trained as drug recognition experts. Results of chemical testing for marijuana, which may be done afterward, may be used as additional evidence of impairment.
A hazardous mix
Driving under the influence of marijuana is risky: It may, researchers say, affect reaction time and coordination. After recreational use was legalized in the state of Washington in 2012, the percentage of drivers involved in fatal crashes who tested positive for marijuana more than doubled, from 8 percent in 2013 to 17 percent in 2014, according to the AAA Foundation for Traffic Safety.
And according to statistics from the National Highway Traffic Safety Administration, the number of weekend drivers involved in nighttime traffic accidents who tested positive for THC increased from just under 9 percent in 2007 to more than 12 percent in 2013-2014. The numbers are likely even higher, since drivers involved in accidents may be impaired by both marijuana and alcohol but tested only for alcohol.
Alarmingly, in a survey of 790 medical marijuana users in Michigan, about half reported that they had driven while “high” in the previous six months. One in five reported driving while “very high.”
Hazy testing
Despite the need for DUI limits for marijuana, establishing uniform standards is difficult for several reasons, including limitations of the current testing methods and debate about what a safe limit (if any) is, since marijuana affects users differently.
- First, it’s not known what kind of test (blood, urine, or saliva) is best. Saliva testing can detect marijuana residues in the mouth, which reflects recent drug use, but this is in its infancy in the U.S. Breath testing is also being developed.
- There is lack of agreement about what blood THC concentration should be considered impairment, since the two don’t necessarily correspond with each other. After smoking marijuana, blood levels of THC drop rapidly, and by the time a traffic accident occurs and the police are involved, THC may no longer be present in the blood—though the chemical can still be having effects in the brain. On the other hand, THC and other marijuana compounds are fat-soluble and can remain in the body’s fat tissues and leach into the blood for weeks beyond when they cause impairment.
- It’s possible that other markers of marijuana use found in the blood, such as cannabinol or cannabigerol, may be better than THC for determining whether someone has recently used marijuana. But the levels may be too low to be detected in blood if marijuana is orally ingested rather than smoked or vaped.
- Urine testing has problems as well: Since marijuana is metabolized in the body before it is excreted in the urine (which can take many hours), a driver under the influence of marijuana may be able to pass a urine test. On the flip side, since marijuana compounds can continue to be excreted in urine for weeks (just as they can be leached into blood for weeks), someone who has not used marijuana recently—and is no longer impaired—could fail the test.
But the biggest wrinkle, perhaps, is that marijuana affects people differently depending on many factors, including how the drug is taken (smoked, vaped, or eaten), how frequently it is used, and percent body fat of the user. And there is no single blood level or saliva concentration of THC (or other marijuana compounds) that can distinguish who is or isn’t impaired.
BOTTOM LINE: No one knows what a “safe” THC level is for driving, how long the impairment persists after using marijuana, or how to best assess legal impairment limits—but as ad campaigns in Colorado have declared, “Drive high, get a DUI” and “Hits Lead to Hits.”
