CBD Users, Driving Is Probably Okay: Study

Smoking Marijuana
Combustibles like the one pictured above are one of the most common consumer marijuana products. AFP / Raul ARBOLEDA

Can smoking CBD affect someone’s driving? Study authors who published in JAMA asked that question. Their 22 study participants smoked and all did inhale: cannabis with CBD, cannabis with CBD and THC, cannabis with THC alone, and placebo. Each participant then completed eight driving tests. The authors measured the drug’s effects by how much the drivers swerved out of lane, at different time periods. 

One question before this story proceeds: Does a sample size of only 22 healthy occasional marijuana smokers really provide any useful data? 

It turns out it does, and that’s because of smart choices the researchers made when designing the study. We’ll be back to these points in a minute. 

Study details 

The researchers wanted to know if there was a difference in impairment between CBD use and THC use. THC is the intoxicating marijuana component. CBD, or cannabidiol, is a chemical belonging to a class of molecules called cannabinoids. In animal studies cannabinoids, are believed to dim the effects of dopamine. THC does just the opposite

This study tested the participants’ driving abilities at different times after they inhaled each of the study drugs, at 40 minutes up to 240 minutes. The results showed a slight increase in lane weaving in the THC and THC plus CBD groups. The difference was greatest measured at the 40-minute and 100-minute marks. 

The CBD-only group had the same amount of lane weaving as the placebo group. 

David L. Streiner, PhD, CPsych, said t he largest difference was between the THC with CBD smokers and the placebo group -- at 1.72 cm, a bit more than one-half inch. “It’s hard to imagine that that represents a driving risk. The biggest differences are in the p erceived ability to drive, not the actual ability,” said Dr. Streiner, who is Professor, Department of Psychiatry & Behavioural Neurosciences, McMaster University and professor, Department of Psychiatry, University of Toronto. He was not part of the study. 

The authors acknowledge that the doses tested may not be the same as those commonly used. CBD products are not regulated, so the amount of CBD and other components in products purchased by the general public may not match what is on the label. The same is true for THC products that are not purchased from cannabis dispensaries. 

Could the study findings reassure CBD users that their driving ability would not be affected by the use of CBD products? Lead author Jan Ramaekers, PhD, Maastricht University, Netherlands, said that although CBD products are not regulated and therefore the exact doses that users get from oral or inhaled CBD products are unknown, there is evidence that driving ability might not be impaired. 

The additional risk is that there have been some discrepancies in labeling of CBD products. Some did contain some THC even though they weren’t supposed to. There was much discussion regarding whether this was an intentional act or mere carelessness. 

Clearly, further research is needed. Not until there is regulation of CBD products due to the possible adulteration with THC and other undisclosed ingredients. This randomized, double-blinded trial -- meaning neither participants nor researchers knew which substance the participant inhaled -- was conducted at Maastricht University. 

Back to the question about study size 

Dr. Streiner told Medical Daily that there were two important things the researchers did when they determined how many subjects were needed for the study. “They did a sample size calculation based on previous research, and they used a cross-over design.” “That’s more powerful than a between-subjects design,” he explained, because it eliminates the differences between people.  

According to the NIH, crossover trials are different because each participant takes every test.  Because each of the 22 subjects inhaled each of the study drugs and participated in all eight driving tests, the results are more powerful than if one person had performed only one driving test. Test variations from one individual to another are decreased and the end results will, theoretically, be more accurate.  

The study was funded by the Lambert Initiative for Cannabinoid Therapeutics at the University of Sydney, Australia. 

Yvonne Stolworthy MSN, RN graduated from nursing school in 1984 and has spent many years in critical care and as an educator in a variety of settings, including clinical trials.

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