As more states legalize marijuana for recreational use, it’s become more common for people to have the drug in their system, leaving public health officials grappling with a thorny question: What constitutes impaired driving?
For alcohol, the cutoff is uniform: A blood alcohol concentration above 0.08 percent. But there is no standard cutoff for THC, the psychoactive compound in marijuana. And even in places that do institute a cutoff, it’s unclear whether exceeding the limit constitutes impairment, experts say.
While cannabis use is associated with cognitive and motor impairment, Dr. Godfrey Pearlson, a professor of psychiatry and neuroscience at Yale University, cautioned that legal limits for THC — which range from 0-5 nanograms per milliliter of blood, depending on the state — “are very loose concepts for which there is very little evidence.”
Blood THC levels typically peak quickly after smoking cannabis, reaching upward of 100 nanograms per milliliter of blood within 15 minutes of smoking it. Then, THC levels drop rapidly to less than 2 nanograms per milliliter of blood after about four hour. It takes around eight hours to reach similarly low concentrations of THC after taking an edible.
A 2019 study led by Dr. Jeff Brubacher, an associate professor in the department of emergency medicine at the University of British Columbia, found that THC concentrations below 5 nanograms per milliliter did not appear to increase the risk of car accidents. Levels above 5 nanograms per milliliter, however, were associated with an increased risk. In Canada, where recreational marijuana was legalized in October 2018, the cutoff for THC is 2 nanograms per milliliter.
In new research published Wednesday in the New England Journal of Medicine, he and his colleagues found that since marijuana was legalized in Canada, the number of drivers involved in accidents with THC levels above the legal limit did indeed increase.
After marijuana was legalized in the country, the number of drivers moderately injured in car accidents who were above the legal limit for THC more than doubled, from less than 4 percent of drivers to 8.6 percent. The percentage of injured drivers above the legal limit for alcohol didn’t change.
Studies in U.S. states, including Washington, where marijuana was legalized in late 2012, have documented similar trends.
But Brubacher said the presence of THC, even at levels that exceed 2 nanograms per milliliter, does not necessarily mean the drivers were impaired and doesn’t mean marijuana use was the cause of the crash.
“The drivers we are most concerned about are those with higher concentrations,” he said. His new study found that the number of moderately injured drivers with concentrations of THC above 5 nanograms per milliliter rose from 1.1 percent before cannabis was legalized to 3 percent after legalization.
“Which isn’t insignificant,” Brubacher said. “But it’s still far less than what we’re seeing for drivers with alcohol above the legal limit. The collision risk for alcohol above 0.08 is sky-high, at around a 500 percent increased risk. With THC, it’s a much smaller risk. It’s not nothing, there is risk, but it’s smaller than with alcohol.”
Timothy Brown, director of drugged driving research and the University of Iowa’s National Advanced Driving Simulator, said that there’s a need for “better research that provides guidance for individuals about when it’s safe to drive.”
While there is relative uniformity in impairment associated with blood alcohol concentrations, THC concentrations are much more challenging to tie to specific cognitive and motor skill performance issues.
“When we use an arbitrary measure of THC in the blood, it doesn’t have the same meaning as a blood alcohol concentration,” Brown said.
One issue is that alcohol is water soluble whereas THC is fat soluble, so using the same type of test for both substances is the best tool law enforcement has at the moment, but may not be reliable, Pearlson said.
The fact that THC may linger at low amounts in the body for longer also presents a challenge for testing daily users.
“How do we differentiate it between someone who is acutely impaired versus someone who just has the drug in their system, between someone who used two hours ago versus someone who used last night and still has residual THC?” Brown said. “Should we be treating them the same? From a traffic safety standpoint, it doesn’t make sense to punish someone if they aren’t making driving unsafe for others.”
Equipping people with the information they need to evaluate whether or not they are driving impaired after using marijuana is key in the absence of a one-size-fits-all test.
“To separate cannabis use from driving, don’t drive if you’re feeling any effects of cannabis,” Brubacher said.
And, he added, while “it’s concerning that the numbers went up, but it shouldn’t take our focus off the biggest issue with impaired driving, which is alcohol.”
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Kaitlin Sullivan is a contributor for NBCNews.com who has worked with NBC News Investigations. She reports on health, science and the environment and is a graduate of the Craig Newmark Graduate School of Journalism at City University of New York.