Medicinal THC impaired real-world driving in both occasional and heavy users
Dronabinol (medicinal THC) impaired on-the-road driving performance in both occasional and heavy cannabis users, but field sobriety tests failed to detect the impairment.
Quick Facts
What This Study Found
Twenty-four participants (12 occasional users, 12 heavy users) received dronabinol (10 mg and 20 mg) or placebo in a crossover design, then drove on actual roads. Occasional users showed significant increases in lane weaving (SDLP, p=0.008) and slower reaction times in car-following (p=0.011).
Heavy users also showed impairment that exceeded the threshold associated with blood alcohol of 0.05%, but to a lesser degree than occasional users, consistent with tolerance. Notably, both groups reported similar levels of subjective "high" despite the behavioral difference.
Critically, the Standard Field Sobriety Test failed to distinguish between dronabinol and placebo conditions in either group, meaning it would not identify these impaired drivers at a traffic stop.
Key Numbers
12 occasional + 12 heavy users. Dronabinol doses: 10 mg, 20 mg. SDLP increase significant (p=0.008) in occasional users. Impairment exceeded BAC 0.05% equivalent in both groups. SFST did not discriminate between conditions.
How They Did This
Double-blind, placebo-controlled, randomized, three-way crossover study. 12 occasional and 12 heavy cannabis users received placebo, 10 mg, or 20 mg dronabinol. Primary outcome was SDLP (lane weaving) in on-the-road driving. Car-following reaction time and SFST performance were secondary measures.
Why This Research Matters
This study used actual on-the-road driving, the most ecologically valid measure of driving impairment. The finding that medicinal THC doses impaired driving more than a BAC of 0.05% was directly relevant to patients prescribed dronabinol.
The Bigger Picture
Patients prescribed medicinal THC face a dilemma: the drug may help their medical condition but impairs driving at therapeutic doses. The failure of field sobriety tests to detect this impairment means standard roadside checks would not protect these drivers or others on the road.
What This Study Doesn't Tell Us
Sample size of 24. Dronabinol (oral THC) has different pharmacokinetics than smoked cannabis. The on-road driving course, while realistic, was conducted under controlled conditions. Tolerance assessment was between-group (occasional vs heavy) rather than within-subject.
Questions This Raises
- ?Should patients prescribed dronabinol be warned about driving impairment?
- ?How long after dosing does driving impairment persist?
- ?Why do field sobriety tests fail to detect clinically meaningful driving impairment from THC?
Trust & Context
- Key Stat:
- Driving impairment exceeded the 0.05% BAC equivalent
- Evidence Grade:
- Well-designed crossover RCT using on-the-road driving, the gold standard for driving impairment research. Moderate sample size.
- Study Age:
- Published in 2012. On-road driving studies remain rare due to safety and logistical challenges.
- Original Title:
- Medicinal Δ(9) -tetrahydrocannabinol (dronabinol) impairs on-the-road driving performance of occasional and heavy cannabis users but is not detected in Standard Field Sobriety Tests.
- Published In:
- Addiction (Abingdon, England), 107(10), 1837-44 (2012)
- Authors:
- Bosker, Wendy M, Kuypers, Kim P C(4), Theunissen, Eef L(8), Surinx, Anke, Blankespoor, Roos J, Skopp, Gisela, Jeffery, Wayne K, Walls, H Chip, van Leeuwen, Cees J, Ramaekers, Johannes G
- Database ID:
- RTHC-00546
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 medicinal THC impair driving?
Yes. This study found that both 10 mg and 20 mg dronabinol doses impaired lane-keeping and reaction times during actual road driving. The impairment was greater than that associated with a blood alcohol level of 0.05%. Heavy users showed less impairment than occasional users, but both exceeded safety thresholds.
Why can't police detect THC impairment with field sobriety tests?
Field sobriety tests were designed to detect alcohol impairment, which affects balance, eye tracking, and coordination in specific ways. THC impairs driving through different mechanisms (attention, reaction time, lane tracking) that the standard tests are not calibrated to detect.
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Cite This Study
https://rethinkthc.com/research/RTHC-00546APA
Bosker, Wendy M; Kuypers, Kim P C; Theunissen, Eef L; Surinx, Anke; Blankespoor, Roos J; Skopp, Gisela; Jeffery, Wayne K; Walls, H Chip; van Leeuwen, Cees J; Ramaekers, Johannes G. (2012). Medicinal Δ(9) -tetrahydrocannabinol (dronabinol) impairs on-the-road driving performance of occasional and heavy cannabis users but is not detected in Standard Field Sobriety Tests.. Addiction (Abingdon, England), 107(10), 1837-44. https://doi.org/10.1111/j.1360-0443.2012.03928.x
MLA
Bosker, Wendy M, et al. "Medicinal Δ(9) -tetrahydrocannabinol (dronabinol) impairs on-the-road driving performance of occasional and heavy cannabis users but is not detected in Standard Field Sobriety Tests.." Addiction (Abingdon, 2012. https://doi.org/10.1111/j.1360-0443.2012.03928.x
RethinkTHC
RethinkTHC Research Database. "Medicinal Δ(9) -tetrahydrocannabinol (dronabinol) impairs on..." RTHC-00546. Retrieved from https://rethinkthc.com/research/bosker-2012-medicinal-9-tetrahydrocannabinol-dronabinol
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.