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May 10, 2021
Does CBD Really Offset THC's Effects? A Landmark Driving Study Finds Clues

BY JESSICA CASTILLO | Photo by Zac Harris

To say that CBD is having a moment might be something of an understatement. The compound, which occurs naturally in cannabis plants, is available in everything from skincare to sparkling water, and celebrities and everyday people all swear by it for stress relief, pain management, and more. But just because the compound is everywhere, doesn’t mean that experts have figured out exactly what kind of effects it might have on the person using them. Thankfully, that’s beginning to change.

A groundbreaking study from researchers at the University of Sydney in Australia and Maastricht University in the Netherlands tested the effects of CBD-dominant cannabis on 26 healthy people as they drove 100 kilometers down the highway in the Netherlands, as well as the effects of THC-dominant cannabis, a type of cannabis with equal percentages THC and CBD, and a placebo. The study, which was published in December 2020, measured each driver’s standard deviation of their lateral position during the drive—crucially, they found that people exhibited roughly the same level of swerving or other driving deviation after they inhaled the CBD-dominant cannabis and the placebo, but swerved more frequently when they inhaled the THC-dominant cannabis and the hybrid blend. 

While the study is a promising start to better understanding how cannabinoids like CBD and THC affect a person’s cognition after using cannabis, there are a number of caveats to keep in mind. Ember talked to Dr. Thomas Arkell, a researcher at the University of Sydney, and Dr. Jan Ramaekers, a professor of psychopharmacology at Maastricht University, to learn more about their study’s findings.

Why are researchers interested in CBD?

As Dr. Arkell noted, there’s a lot of research being done regarding CBD right now, but at present there haven’t been as many high-quality clinical trials that center on how CBD impacts people’s cognitive ability. This trial could be the beginning of a lot of literature that changes that.

“CBD is everywhere, and it's being used more and more,” he told Ember, noting that it’s still more widely available in the United States than it is in other countries like Australia. “With people using it all the time and potentially in high doses, we wanted to know what implications that might have for driving and cognitive performance, whether the dose of CBD has any effects at all on people or whether it's a fairly benign substance in terms of producing any impairment or intoxication,” he said.

In particular, the researchers were interested in seeing if there was any truth to the commonly-held belief that CBD offsets the effects of THC—that is to say, that inhaling or consuming a hybrid strain of cannabis would mitigate THC’s side effects. Given that it is still notoriously difficult to conduct clinical trials centering cannabis in the United States, researchers often rely on conducting their studies in countries where it is possible—though while they could legally allow trial participants to drive in the Netherlands, the researchers working on the Maastricht University study were still overseen by an independent review board.

“There's a long history of research on the negative effects and the impairing effects of THC on cognitive performance and neurocognition that has been conducted previously, but there is still a kind of debate on how bad these effects actually are,” Dr. Ramaekers told Ember. “What is the magnitude of these effects? How serious are they and how long do they last? This,” he noted, “is particularly relevant in the context of driving a car”—as well as other everyday activities that people might try to engage in after consuming cannabis.

“Particularly in the United States, I think you're in a prime position of getting from a war on drugs to how to live with drugs,” he said. “What we really need is to be open and frank about the benefits and risks of drugs including cannabis. I think both aspects are important and should be told in a very balanced way.”

How researchers conducted the study

Researchers called in their 26 test subjects, all of whom underwent a double-blind study on four different occasions. They were asked to vape one of three strains of cannabis, plus a placebo—this was a way to control the dose of cannabis that people consumed, given that vaping can be more tightly monitored than smoking flower through a joint or pipe, and is more regulated than many edibles.

Neither the participants nor the researchers knew which strain of cannabis they vaped on each trial, or if they vaped the placebo. Approximately 40 minutes after inhaling their dose, they got in a car equipped with a second steering wheel for a driving instructor; they then replicated the drive four hours after their first inhalation. 

Not all of the drivers completed every drive, but those who did exhibited roughly similar levels of erratic driving, such as swerving, lane weaving, and overcorrecting. When drivers drove 40 minutes after inhaling the THC-dominant cannabis and the cannabis with equal levels of THC and CBD, they swerved roughly the same amount; when they inhaled the CBD-dominant cannabis and the placebo, they also swerved roughly the same amount. The drive conducted four hours after inhaling mimicked these ratios, though the effects of THC seemed to have worn off. 

These findings suggest that CBD may not impair cognitive ability in the way that THC does, though it does not offset the effects of THC the way many people believe it will. Though studies are limited and the methods vary, the risks associated with driving under the influence of THC alone haven’t been “consistent,” the advocacy group NORML noted—though there is definitely a risk involved when people consume cannabis and alcohol simultaneously, as people commonly do. 

“The fact that CBD didn't produce any impairment is not totally surprising,” Dr. Arkell said. “A lot of people have said, ‘Didn’t we already know that? And I was like, ‘Well, we thought that, but that's why we do science like this to confirm if that actually is the case.” Dr. Ramaekers added that the fact that researchers can replicate these results over and over again is a useful indicator for future studies.

Does this mean it’s safe for me to drive after using CBD?

The researchers would strongly caution against this, even if you feel fine.

To start, you’re probably not driving a car with a secondary emergency steering function, nor are you likely driving with a licensed safety instructor in the passenger seat at all times. In many states, it’s illegal to drive a car while high, so even if you feel fine, it’s important to wait four to five hours before getting behind the wheel. 

“It’s important to keep in mind that if you're unsure of your driving ability or you feel like you shouldn't be driving, that's a pretty good sign that you shouldn't be driving,” Dr. Arkell said. “People do need to keep that in mind and not overestimate their driving ability,” even after taking over-the-counter CBD.

“What we need to do is to inform the public on the pros and cons of cannabis use, but not inform them incorrectly,” Dr. Ramaekers echoed. “Let’s just be open and frank about it. I think one of these conclusions should be that people should not drive when they are actually in the intoxication phase. That does not mean that they should not smoke cannabis if they want to. That's fine. But let's take into account that if you did, please don't get into your car for the next four hours so at least then you know that you will be safe in your car and you will not jeopardize your own life, but also the life of others that also are using public highways.”

So what does this study mean for the future of CBD?

It’s a groundbreaking study to be sure, but it’s also just the beginning of more research, especially given that it was tightly controlled and participants only inhaled a single low dose of cannabis containing both THC and CBD, at 13.75 milligrams each. Many CBD products, whether medicinal or over-the-counter, boast far more CBD than that, and it’s tough to say exactly how much these products contain, especially if they come from sources without as much regulation. 

The study “does show that, in principle, CBD as a compound is not likely to affect driving performance, but of course we need a bit more resource to actually go to the highest those levels as well,” Dr. Ramaekers said

Dr. Arkell agreed. “There's certainly a lot more research to go,” he said, adding that the study “points us in the direction of looking at higher CBD doses and oral CBD doses, and seeing whether those produce any effects by themselves or when combined with THC.” He noted that researchers are ultimately “trying to catch up with the way that people are using cannabis in the real world. If that's people using 200-milligram CBD gummies, then let's test 200-milligram CBD gummies and see what that's doing.”

It’s also worth keeping in mind that the way you ingest cannabis or CBD will greatly affect your dosage, as well as how quickly you’ll begin feeling the effects. The researchers used vapes in their study because they could control the dose each participant received, and accounted for things like the length of a puff or the side stream as someone exhales—two key components in inhaling a cannabis cigarette or pipe hit. Edibles take longer to go through your system, and light up the cannabinoid receptors in your brain in a different way to inhalation, and the study did not look at how that might affect someone’s impairment.

Ultimately, Dr. Arkell hopes people reading the study finally do away with the myth that CBD will offset any effects of THC—that just doesn’t seem to be the case, no matter how good you feel.

“We did see a slight decrease in how stoned people felt when the CBD was combined with the THC, but people actually had higher THC concentrations in their blood,” he said. “If people are going in with the expectation that combining CBD with THC may reduce any unwanted side effects or reduce some of the impairing effects of THC, they should keep in mind that even though they might feel like that, it doesn't necessarily mean that it's actually having those effects.” 

These statements have not been evaluated by the Food and Drug Administration. Products are not intended to diagnose, treat, cure, or prevent any disease.
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