In a controlled trial, cannabis with equal parts CBD and THC impaired driving and cognition just as much as THC-dominant cannabis, and in some tasks, the CBD-containing cannabis actually worsened impairment.
Cannabis consumers, driving safety researchers, regulators developing impaired driving policies, and anyone who believes CBD counteracts THC.
CBD did not reduce impairment
What the researchers found
Both THC-dominant and THC/CBD equivalent cannabis increased lane weaving during simulated driving. THC/CBD equivalent cannabis actually produced worse impairment on divided attention and auditory processing tasks than THC-dominant cannabis. Peak plasma THC concentrations were higher with THC/CBD cannabis, suggesting a pharmacokinetic interaction where CBD increases THC absorption.
Why it matters
Many consumers and regulators assume CBD mitigates THC impairment, which influences product labeling, driving policies, and consumer choices. This study challenges that assumption directly.
The numbers in context
14 participants. 125 mg cannabis vaporized per session. 11% THC in both active conditions. THC/CBD cannabis produced worse performance on Divided Attention Task and Paced Auditory Serial Addition Task. Peak plasma THC was higher with THC/CBD cannabis. Subjective "stoned" ratings did not differ between active conditions.
How the study worked
Randomized, double-blind, within-subjects crossover design with 14 healthy light cannabis users. Three conditions: THC-dominant (11% THC, <1% CBD), THC/CBD equivalent (11% THC, 11% CBD), or placebo. Simulated driving and cognitive testing at 20-60 min and 200-240 min post-vaporization.
What this study cannot tell us
Small sample (n=14) of light cannabis users. Simulated driving may not perfectly represent real-world conditions. Only one CBD:THC ratio was tested. Acute dosing does not capture chronic use patterns.
How to read the evidence
Rated moderate because this is a randomized, double-blind crossover trial, though the sample size is small.
When this study was published
Published in 2019. Cannabis driving impairment research has continued to expand as more jurisdictions legalize.
The bigger picture
The belief that CBD counteracts THC is widespread but may be misleading when it comes to functional impairment. If CBD actually increases THC blood levels, "balanced" products marketed as safer could paradoxically increase impairment risk.
Questions still open
- Does CBD consistently increase THC blood levels through pharmacokinetic interaction? Would higher CBD:THC ratios show different results? Should driving regulations distinguish between THC-dominant and balanced cannabis products?
Common questions
Does CBD cancel out THC impairment?
Why might CBD make impairment worse?
Is it safer to drive after using balanced CBD/THC products?
Read the original research
Cannabidiol (CBD) content in vaporized cannabis does not prevent tetrahydrocannabinol (THC)-induced impairment of driving and cognition.
Psychopharmacology, 236(9), 2713-2724
Citation
Arkell, Thomas R; Lintzeris, Nicholas; Kevin, Richard C; Ramaekers, Johannes G; Vandrey, Ryan; Irwin, Christopher; Haber, Paul S; McGregor, Iain S. (2019). Cannabidiol (CBD) content in vaporized cannabis does not prevent tetrahydrocannabinol (THC)-induced impairment of driving and cognition.. Psychopharmacology, 236(9), 2713-2724. https://doi.org/10.1007/s00213-019-05246-8