As cannabis legalization spreads across North America, researchers and policymakers are scrambling to solve the issue of road safety. Drunk driving remains a higher risk for accidents, but recent data suggests cannabis-related road incidents are on the rise.
In the past, studieshave linked smoking cannabis to slower driving, delayed reaction time on the road, coordination issues and impaired decision making. The latest evidence, published on JAMA Network in January, looked at drivers over 65 — one of the fastest-growing age groups of cannabis users — and found more reason for concern. Participants showed significant impairment 30 minutes after smoking or vaping. And after three hours, when blood THC levels were low, they still felt too stoned to drive safely.
“Their weaving went up and they didn't stay in the lane as well,” the study’s co-author Patricia Di Ciano, a University of Toronto professor and researcher at Canada’s Centre for Addiction and Mental Health, tells Yahoo Life. “The participants told us they weren’t capable of driving so they also slowed down on the road as a way to compensate.”
Experts say smoking or vaping cannabis can impair drivers for at least four hours after their last puff. But what about edibles? “The whole issue of edibles and driving is incredibly understudied,” Dr. Godfrey Pearlson, professor of psychiatry and neuroscience at Yale University school of medicine and a leading researcher of cannabis and driving, tells Yahoo Life. That’s because federal drug regulations prevent researchers from using consumer-grade edibles in clinical trials (at least, for now).
But with more people using edibles for symptom relief and a smoke-free way to get high, there are a lot of questions surrounding when it’s safe to get behind the wheel.
How long should you wait to drive after eating an edible?
That’s not well understood. “It's not clear when edibles are going to kick in or how long they’re going to last so that's much more of a risky proposition than consuming cannabis in other ways,” says Pearlson.
While the risk of impairment after smoking weed subsides around four hours, Pearlson says that edibles take twice that amount of time — or longer. “There’s a more complex absorption and reabsorption loop that happens when people take THC edibles, then when they smoke it,” he explains.
The problem, particularly with edibles, is that different people have different tolerance issues, depending on how often they use the drug, their age and other biological factors. For example, “older people have higher risk for cognitive vulnerability at baseline and they metabolize many drugs including THC differently than younger people because of their body composition levels,” points out Pearlson.
Even eating a gummy on a full stomach can delay or prolong the effect of an edible. However, in general, most experts advise that if you’re going to eat a gummy, you should wait anywhere from eight to 12 hours before getting behind the wheel.
Is there a certain amount you can eat and still drive?
That’s another tricky question. Unlike the blood alcohol limit — which is easier to define and detect with roadside tests — cannabis intoxication isn’t as clear cut. “The general rule is that higher doses result in more impairment for a longer period of time,” says Pearlson. “The problem with edibles is that different people have different tolerances.”
Studies have shown that tolerance can play a role in how quickly you return to baseline. So infrequent users can be very much impaired by a low dose, while a daily user might have less adverse response on a larger dose. Another problem, despite the dosage written on a brand’s packaging, is that the actual amount of THC varies from product to product. This makes it even harder to pin down how much is too much.
That hasn’t stopped lawmakers from setting limits in an attempt to crack down on accidents. Cannabis intoxication limits vary by state — anywhere from zero to 5 nanograms per milliliter of blood. But most clinical researchers say those numbers don’t correlate with impairment. “What’s happening in the blood doesn’t always reflect what’s happening in the brain,” says Di Ciano. In her study, participants who showed the biggest driving impairment didn't necessarily have the highest blood levels of THC.
Unlike alcohol, which is water soluble and evenly distributed through your blood, THC is fat soluble, which means it’s harder to test for in blood or breath. With edibles, THC concentrations in blood are even less reliable predictors. A 2022 study published in the Journal of Psychopharmacology found that participants given oral cannabis (the lab-grade alternative to an edible) showed low blood THC concentration at the height of their driving impairment.
Conversely, regular cannabis users — particularly medical cannabis users — may clock in at higher concentrations, even if they’re not impaired. Because marijuana stays in your system long past the window of cognitive impairment, the potential for false arrests is an increasing concern in states where cannabis is legal. “As of right now, there are no good biological methods of determining who might be impaired,” says Thomas D. Marcotte, professor of psychiatry at University of California San Diego, tells Yahoo Life.
Marcotte runs the Center for Medicinal Cannabis Research where he studies how the drug impacts drivers. One thing he’s found is that after the immediate effects of cannabis wear off, people misjudge how impaired they are. “You're sort of always comparing it to how you felt a little while ago,” says Marcotte. “So that's a really concerning point — people start thinking they're doing better when in fact they still shouldn’t be driving.“
In the past few years, apps like Druid have emerged to help assess how stoned is too stoned to drive. Druid’s app is free and features 1- and 3-minute tests to measure your cognitive and motor skills. (Di Ciano suggests taking the tests when you’re unimpaired to get a baseline.)
But what about people who use cannabis to help with health conditions that affect their driving?
In one of Marcotte’s studies, he looked at drivers with multiple sclerosis who used THC to manage spasticity. “In some cases, people may be more functional, they may drive better and perhaps improve certain aspects of driving, like their ability to work the pedals,” he says. But he emphasizes more research is needed and ratios would need to be refined to find what he calls “the sweet spot” of symptom improvement with limited psychoactive effects.
In the meantime, experts suggest staying off the road after eating that gummy. And for those using edibles routinely to manage symptoms for a health condition, it’s important to know exactly how a specific product and dosage affect their abilities. “Never drive after taking edibles if it’s your first time trying that particular brand and dose,” advises Pearlson. “You don't know how that edible is going to affect you until you’ve tried it a few times.”
Adds Marcotte: “Obviously, the best solution is time. I know that's not always practical, but wait it out — that's the safest thing to do.”