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Driving performance was impaired when healthy young people inhaled vaporized cannabis with Δ⁹-tetrahydrocannabinol (THC), but not cannabis that was cannabidiol (CBD)-dominant, a small randomized clinical suggested.

At 40 to 100 minutes after vaping, the standard deviation of lateral position (SDLP) -- a measure of lane weaving, swerving, and overcorrecting -- was increased by THC-dominant cannabis (+2.33 cm, 95% CI 0.80-3.86, P<0.001) and THC/CBD-equivalent cannabis (+2.83 cm, 95% CI 1.28-4.39, P<0.001), but not CBD-dominant cannabis (-0.05 cm, 95% CI -1.49 to 1.39, P>0.99), compared with placebo, reported Jan Ramaekers, PhD, of Maastricht University in the Netherlands, and co-authors in .

"Cannabis-induced driving impairment varies with cannabis strains," Ramaekers noted. "Strains that are rich with THC cause driving impairment, but strains that contain CBD and no THC do not," he told 番茄社区. "This is important as CBD strains may be prescribed for the treatment of medical conditions." There's also a popular notion that CBD counteracts THC's psychoactive effects, which this study shows is not the case, he added.

The findings do not support the conclusion that it's safe to drive after consuming CBD, said JAMA associate editors Thomas Cole, MD, MPH, and Richard Saitz, MD, MPH, in an .

"Consumption of CBD-dominant cannabis did not impair driving in this study, but the authors acknowledged that the doses tested may not represent common usage and the effect size for CBD-dominant cannabis may not have excluded clinically important impairment," they wrote.

The study involved 26 healthy occasional cannabis users, who tested THC-dominant cannabis (13.75 mg of THC), CBD-dominant cannabis (13.75 mg of CBD), THC/CBD-equivalent cannabis (13.75 mg of both), and placebo.

Medical cannabis and CBD are used in many conditions, as a sleep aid or to help manage pain and/or symptoms in neurological disorders like Parkinson's disease, for example. In the U.S., pharmaceutical-grade CBD has been approved by the FDA for three rare pediatric epilepsies: Dravet and Lennox-Gastaut syndromes and tuberous sclerosis complex. CBD doses used in pediatric epilepsy often are around 10 to 20 mg/kg, considerably higher than what was tested in the trial.

"It's a low dose indeed, compared to pharmaceutical grade CBD," Ramaekers said. "In cannabis strains that are presently sold on the free market, the amount of CBD one would consume after a single cannabis cigarette, however, is relatively low; it's comparable to what was dosed in the present study. We focused on THC and CBD consumption that would primarily reflect recreational use of cannabis."

But the THC dose studied also may not be what's in a cannabis cigarette. "Cannabis strains vary considerably in terms of percentage THC," Ramaekers noted. "In other words, there is no typical cannabis joint. The higher the concentration, the less one needs to smoke to achieve a desired high. In the present study, we gave a dose that produced a normal-to-strong feeling of subjective high."

The crossover trial included four experimental sessions -- CBD, THC, THC/CBD, and placebo -- scheduled at least 1 week apart. Participants were 23 years old on average and reported using cannabis less than twice a week in the previous year but more than 10 times in their lives. They had been driving at least 2 years and had normal weight.

In each session, participants waited 40 to 100 minutes after vaping, then drove a car over a 100-km highway circuit while maintaining a constant speed of 95 km/hour (59 mph) and a steady lateral position in the right (slower) traffic lane. They were instructed to drive in the middle of the lane, allowing about 1 meter of road surface on either side of the vehicle. The vehicle had dual accelerator and brake pedals that a licensed driving instructor who accompanied them could operate.

Mean SDLP was calculated by summing deviations in lateral position over the time of the driving test. Lateral position, the distance between the vehicle and the lane boundary to its left, was recorded by a camera mounted on the roof of the car. During driving tests, the range of lateral position values was approximately 54 cm. At 240 to 300 minutes after vaping, SDLP did not differ significantly for any group compared with placebo (P=0.20).

"THC-dominant and THC/CBD-equivalent cannabis produced a short-term impairment during experimental on-road driving, as indexed by a significant increase in SDLP measured 40 to 100 minutes following vaporization," the researchers wrote. "In agreement with previous studies involving smoked cannabis or oral THC (dronabinol), this impairment was modest in magnitude and similar to that seen in drivers with a 0.05% blood alcohol content (≈2.4-2.5 cm)."

"Drivers who consumed THC were generally aware that their driving was impaired, although participants reported that consumption of THC/CBD was associated with less anxiety, reduced strength of drug effects, and greater confidence to drive than THC alone," the editorialists observed. "These findings challenge the myth that CBD ameliorates the psychoactive/psychomotor effects of THC."

"Clinicians should caution their patients that cannabis products containing equal parts CBD and THC are no less impairing than products containing THC alone," Cole and Saitz added. "Moreover, given that alcohol is a major preventable cause of motor vehicle crash deaths and risk is additive with cannabis, patients should be advised to avoid any drinking, particularly with cannabis use, before driving."

The study's limitations include its small sample size. Participants may not be representative of people who use medicinal CBD or regularly use recreational cannabis. In addition, only one dose of CBD, one dose of THC, and a single 1:1 ratio of CBD and THC were tested. The dose tested corresponds with the lower end of the range of CBD concentrations (0.10 mg/mL-655.27 mg/mL) observed in a sample of extracts sold online, the editorialists noted.

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