Adding CBD to cannabis did not reduce THC-related driving and cognitive impairment
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.
Quick Facts
What This Study 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.
Key Numbers
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 They Did This
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.
Why This Research 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 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.
What This Study Doesn't 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.
Questions This Raises
- ?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?
Trust & Context
- Key Stat:
- CBD did not reduce impairment
- Evidence Grade:
- Rated moderate because this is a randomized, double-blind crossover trial, though the sample size is small.
- Study Age:
- Published in 2019. Cannabis driving impairment research has continued to expand as more jurisdictions legalize.
- Original Title:
- Cannabidiol (CBD) content in vaporized cannabis does not prevent tetrahydrocannabinol (THC)-induced impairment of driving and cognition.
- Published In:
- Psychopharmacology, 236(9), 2713-2724 (2019)
- Authors:
- Arkell, Thomas R(10), Lintzeris, Nicholas(19), Kevin, Richard C(16), Ramaekers, Johannes G, Vandrey, Ryan, Irwin, Christopher, Haber, Paul S, McGregor, Iain S
- Database ID:
- RTHC-01919
Evidence Hierarchy
Participants are randomly assigned to treatment or placebo groups to test cause and effect.
What do these levels mean? →Frequently Asked Questions
Does CBD cancel out THC impairment?
Not in this study. Cannabis with equal CBD and THC caused the same or worse driving and cognitive impairment compared to THC-dominant cannabis.
Why might CBD make impairment worse?
The study found higher peak THC blood levels with THC/CBD cannabis, suggesting CBD may increase THC absorption. Higher THC levels could explain the greater cognitive impairment on some tasks.
Is it safer to drive after using balanced CBD/THC products?
This study found no safety advantage. Both types of cannabis increased lane weaving and impaired attention, with balanced products performing worse on some cognitive measures.
Read More on RethinkTHC
Cite This Study
https://rethinkthc.com/research/RTHC-01919APA
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
MLA
Arkell, Thomas R, et al. "Cannabidiol (CBD) content in vaporized cannabis does not prevent tetrahydrocannabinol (THC)-induced impairment of driving and cognition.." Psychopharmacology, 2019. https://doi.org/10.1007/s00213-019-05246-8
RethinkTHC
RethinkTHC Research Database. "Cannabidiol (CBD) content in vaporized cannabis does not pre..." RTHC-01919. Retrieved from https://rethinkthc.com/research/arkell-2019-cannabidiol-cbd-content-in
Access the Original Study
Study data sourced from PubMed, a service of the U.S. National Library of Medicine, National Institutes of Health.
This study breakdown was produced by the RethinkTHC research team. We analyze and report published research findings without making health recommendations. All interpretations are based solely on the published abstract and study data.