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.
Read this if you take medicinal THC and want to understand the driving risk.
Driving impairment exceeded the 0.05% BAC equivalent
What the researchers 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.
Why it 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 numbers in context
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 the study worked
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.
What this study cannot 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.
How to read the evidence
Well-designed crossover RCT using on-the-road driving, the gold standard for driving impairment research. Moderate sample size.
When this study was published
Published in 2012. On-road driving studies remain rare due to safety and logistical challenges.
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.
Questions still open
- 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?
Common questions
Does medicinal THC impair driving?
Why can't police detect THC impairment with field sobriety tests?
Read the original research
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
Citation
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
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