Strains rich with THC, the chemical that produces marijuana’s “high,” cause driving impairment up to 4 hours after vaping, but strains that contain CBD (cannabidiol) and no THC do not, says researcher Johannes Ramaekers, PhD, professor of psychopharmacology and behavioral toxicology at the University of Maastricht in the Netherlands.
"The implication for the general public is that the cannabis-induced driving impairment should be acknowledged as a public health risk, while taking into account that impairment may differ between cannabis strains and depends on time [elapsed] after use," he says.
The study, published in the Journal of the American Medical Association, was small with just 26 people.
CBD use has skyrocketed in the U.S. According to a 2019 Gallup poll, 1 in 7, or 14%, of Americans say they use CBD products in the form of edibles, oils, tinctures, and topicals, often to relieve pain, anxiety or to improve sleep.
CBD is one of more than 100 cannabinoids found in marijuana, along with THC. While some call CBD non-psychoactive, other experts say a better term is that it is ''non-intoxicating."
The Dutch researchers conducted a study that was more ''real life" than would be approved in the U.S. They tested the participants driving on a highway rather than with a driving simulator.
They enrolled 26 healthy men and women, on average 23 years old, who reported cannabis use less than twice a week in the last year but more than 10 times in their lives. None were taking medicines known to affect driving. In all, 22 completed the entire study.
Each participant drove a vehicle equipped with a camera on the roof. For each of the four sessions, they drove twice for about an hour on a highway near Maastricht -- once about 40 to 100 minutes after vaping and then about 240 to 300 minutes after vaping one of four preparations:
- 13.75 milligrams of THC
- 13.75 milligrams of THC/CBD
- 13.75 milligrams of CBD
- A placebo
While there is not consensus on a ‘’typical” dose of marijuana for a recreational user, it may be about 0.5 grams, says Paul Armentano, deputy director of NORML, an organization that promotes marijuana legalization. But, he says, "More important than dose is whether or not the user is relatively naive or habitual, as tolerance plays a major role in drug effect.”
Researchers looked at a measure that detects lane weaving and evaluated average speeds and range of speeds. They took blood samples to look at levels of cannabinoids.
Participants self-rated the effects after each session, including confidence in driving. Researchers evaluated memory, decision making, thinking skills, and other measures after the drives.
Results: THC vs. CBD
The cannabis that was THC-dominant and the THC/CBD combination produced short-term impairment, as measured 40 to 100 minutes after the session. The impairment was similar to that seen in drivers with a blood alcohol content of 0.05%. A blood alcohol content of .08% while driving is defined as a crime in all U.S. states but Utah, where the limit is .05%.
Drivers' lane weaving was no different after taking placebo or CBD only, the researchers say, indicating that CBD, as it was given in the study, ''did not impair driving."
The CBD-dominant cannabis also did not affect thinking or driving skills compared with placebo.
"Don’t drive for at least 4 hours after smoking a cannabis product that contains THC as well as CBD,” Ramaekers says. If the cannabis product contains a low dose of CBD and no THC, then the impact on driving is minimal. He stressed that he is only talking only about the dose tested.
"The results of this small study are relatively unsurprising," says Armentano. And even though the study was small, he said the findings are consistent with previous studies.
"CBD is not psychoactive in the same manner as THC and thus one would not anticipate that it would significantly impact driving performance." He points to previous research on the CBD prescription Sativex, a THC/CBD spray to relieve spasticity in multiple sclerosis. The researchers, including an employee from the drugmaker, found no effect on driving performance after evaluating studies and registries from 2000 to 2017.
Adrian Devitt-Lee, chief science writer for Project CBD, a nonprofit promoting research about medical uses of CBD, agrees that the study results aren't surprising but says it could have gone further. The researchers might have studied if those who vaped THC drove slower, ''as they usually do." Also worthy of more study: how long it takes for the effects of THC to wear off completely.
Another issue not addressed in the study, Armentano says, is whether the changes in driving behavior affect the risk of an accident. A recent study found the number of drivers who tested positive for marijuana involved in fatal crashes in Washington state doubled from 2013 to 2017, after legalization of marijuana.