Drivers who consume cannabis gummies are impaired for hours after taking the drugs, according to a new study published Monday.
Researchers gave regular cannabis users either an edible or a placebo gummy, then had them operate a driving simulator. While the researchers did not see a meaningful effect among those who took just a 2 milligram dose of THC, the psychoactive compound in cannabis, those who consumed 10 milligrams wove in and out of their lanes significantly more than those who were given placebo gummies. The effects were even more pronounced after 20 milligrams, when study participants also had slower reaction times and drove at erratic speeds.
“The data clearly show that it’s affecting driving ability, and the more you take, the more the impact,” said Dr. Bernard Le Foll, the lead author of the paper, which was published in JAMA Network Open. “This is very important, because lots of people I talk to who are users deny this or will say, ‘Oh, sometimes I drive even better when I use cannabis,’” added Dr. Le Foll, a senior scientist at the Institute for Mental Health Policy Research at the Centre for Addiction and Mental Health in Toronto.
The drivers were impaired two hours after they ingested edibles, and were still impaired five hours after consuming them. “The impact is prolonged,” Dr. Le Foll said.
Cannabis edibles have become increasingly popular as more states have legalized recreational use. Doses of 2.5 milligrams or less are generally considered low and nonintoxicating, with psychotropic effects starting at around 5 milligrams. More experienced users may take 10 milligrams, with doses of 20 milligrams generally considered to be on the higher end for users who have some tolerance.
The study was small, like most randomized controlled trials of this type. It included 40 men and women aged 19 to 45 who used cannabis regularly for nonmedical reasons. The authors noted that the findings may differ in younger or older adults, or in people who don’t use cannabis or use it for medical reasons. They also noted that the driving scenario was a straightforward, rural drive and the results could differ in more heavily trafficked urban areas.
It is well-documented that drinking alcohol and smoking marijuana can both impair driving, but few studies have examined the effects of edibles. Another recent study, published in May, looked at how cannabis edibles and alcohol affect driving, both separately and in combination. It found that using low doses of the substances together led to greater driving impairment than a small amount of either alone.
Tory Spindle, the senior author of that study and an associate professor of psychiatry and behavioral sciences at Johns Hopkins University School of Medicine, said that the impairment seen with a few beers and a THC gummy together was equivalent to the effects of having a blood alcohol concentration of 0.08 percent, the legal limit in most states.
That study also found impaired driving among subjects who consumed 10 milligrams of THC alone, but the effect fell just short of reaching statistical significance.
José Ignacio Nazif-Munoz, an associate professor in the department of community health sciences at Université de Sherbrooke in Quebec, said it’s important for users to understand that edibles have both a delayed effect as well as a prolonged one.
They don’t lead to intoxication right away. But they “will have an effect eventually, when you go to work, or decide to drive,” said Dr. Nazif-Munoz, who wrote an editorial accompanying the new study. And once that effect kicks in, it can last for hours.
The study also found that impairment occurred at blood THC concentration levels that are below commonly used legal thresholds. (The levels allowed for driving vary from state to state.) That complicates the use of blood levels to identify impaired drivers who consumed edibles, and presents a challenge to law enforcement, Dr. Nazif-Munoz said.
Unlike blood alcohol concentration, which reflects a person’s degree of impairment, blood THC levels are not a good predictor of impairment, experts said. THC can show up in the blood when a person who took an edible is not yet impaired, or after that person is no longer impaired, Dr. Nazif-Munoz added.
“With alcohol, we have certain ideas of when we should stop drinking,” Dr Nazif-Munoz said. With cannabis edibles, he said, “things get a little more complicated.”
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