In a controlled study of 20 adults, standard field sobriety tests showed little sensitivity to cannabis impairment, while a novel app-based test (DRUID) reliably detected impairment from both oral and vaporized cannabis.
Law enforcement, traffic safety researchers, cannabis policy legislators
Blood THC levels returned to baseline before cannabis impairment actually subsided
What the researchers found
High-dose oral and vaporized cannabis impaired cognitive and psychomotor performance, but field sobriety tests showed little sensitivity to cannabis-induced impairment. The DRUID app was the most sensitive test, detecting significant differences from placebo for both administration routes. Blood THC levels returned to baseline before impairment resolved.
Why it matters
Current roadside methods for detecting cannabis impairment are inadequate. Blood THC levels and field sobriety tests both fail to reliably identify when someone is actually impaired, creating significant public safety and legal challenges.
The numbers in context
20 participants (10 men, 10 women); 6 sessions; low doses produced subjective effects but no measurable impairment; high doses impaired performance; DRUID detected impairment at both routes; women showed more DRUID impairment at high vaporized dose; blood THC returned to baseline before impairment resolved.
How the study worked
Double-blind, placebo-controlled, six-session outpatient study with 20 infrequent cannabis users (10 men, 10 women) receiving three oral doses (0, 10, 25 mg THC brownies) and three vaporized doses (0, 5, 20 mg THC), tested with computerized tasks, DRUID app, and field sobriety tests.
What this study cannot tell us
Small sample size (20 participants); infrequent users only (tolerance effects not captured); lab setting differs from real-world driving; limited dose range; DRUID validation still early.
How to read the evidence
Small but well-designed double-blind RCT with placebo control, limited by sample size and lab setting.
When this study was published
Published in 2021.
The bigger picture
The disconnect between blood THC concentrations and actual impairment undermines per se legal limits for THC, while the failure of field sobriety tests suggests entirely new approaches are needed for roadside cannabis impairment detection.
Questions still open
- Could the DRUID app or similar tools replace field sobriety tests for cannabis at roadside? Should per se THC blood limits be abandoned in favor of impairment-based assessments?
Common questions
Can police tell if someone is high while driving?
Does blood THC level indicate impairment?
Read the original research
Assessment of cognitive and psychomotor impairment, subjective effects, and blood THC concentrations following acute administration of oral and vaporized cannabis.
Journal of psychopharmacology (Oxford, England), 35(7), 786-803
Citation
Spindle, Tory R; Martin, Erin L; Grabenauer, Megan; Woodward, Thomas; Milburn, Michael A; Vandrey, Ryan. (2021). Assessment of cognitive and psychomotor impairment, subjective effects, and blood THC concentrations following acute administration of oral and vaporized cannabis.. Journal of psychopharmacology (Oxford, England), 35(7), 786-803. https://doi.org/10.1177/02698811211021583