Oral cannabis significantly impaired occasional smokers on roadside sobriety tests, with 6.4 times higher odds of impairment, while inhaled cannabis effects had dissipated by the time testing occurred 1.5-3.5 hours later.
Read this if you consume cannabis edibles and want to understand how long impairment lasts and when it peaks.
Occasional users had 6.4x higher odds of sobriety test impairment after oral cannabis
What the researchers found
This controlled study compared impairment from smoked, vaporized, and oral cannabis on standard roadside sobriety tests used in driving under the influence evaluations.
For inhaled cannabis (smoked and vaporized), no significant impairment was detected because testing occurred 1.5 and 3.5 hours after dosing, by which time peak effects had subsided.
Oral cannabis (edibles) told a different story. Because edible THC is absorbed slowly, blood concentrations were still peaking during the 1.5-3.5 hour testing window. Occasional smokers who consumed oral cannabis had 6.4 times higher odds of showing two or more impairment clues on the one-leg stand and walk-and-turn tests compared to placebo.
Among all participants, oral cannabis caused significantly larger pupils under direct lighting at both 1.5 and 3.5 hours post-dose. Blood THC and its metabolite 11-hydroxy-THC both independently predicted impairment on these tests.
Frequent cannabis users showed less impairment than occasional users, consistent with tolerance development.
Why it matters
As edible cannabis use increases, understanding its impairment timeline is critical for driving policy. Edibles produce peak impairment 1.5-3.5 hours after consumption, a delayed window that may catch users off guard if they drive thinking the effects have not yet started.
The numbers in context
Oral cannabis: 6.4x higher odds of impairment in occasional users (95% CI 2.3-18.4). THC blood concentration OR for impairment: 1.3 per unit increase. 11-OH-THC OR: 1.5. Pupil dilation: 0.4-0.5 mm larger than placebo.
How the study worked
Controlled study comparing frequent and occasional cannabis smokers after placebo, smoked, vaporized, and oral cannabis (6.9% THC, ~50.4 mg). Modified Romberg balance, one-leg stand, and walk-and-turn tests administered at 1.5 and 3.5 hours. Pupil sizes measured under direct lighting.
What this study cannot tell us
Small sample size limits generalizability. Testing occurred at fixed time points that favored detecting oral but not inhaled impairment. The standardized field sobriety tests were designed for alcohol impairment and may not optimally detect cannabis effects. Only one dose was tested.
How to read the evidence
Controlled experimental study with placebo comparison and multiple administration routes. Moderate because it demonstrates effects in a controlled setting with clinically relevant endpoints.
When this study was published
Published in 2017.
The bigger picture
These findings are directly relevant to cannabis-impaired driving enforcement. Standard field sobriety tests can detect oral cannabis impairment in occasional users, but the delayed onset means drivers may be impaired well after consuming an edible, unlike alcohol where impairment onset is more predictable.
Questions still open
- Should DUI enforcement develop cannabis-specific sobriety tests? How long should users wait after consuming edibles before driving? Could blood 11-OH-THC levels serve as a more reliable impairment biomarker than THC itself?
Common questions
How long after eating an edible are you impaired?
Can police field sobriety tests detect cannabis impairment?
Read the original research
Evaluation of divided attention psychophysical task performance and effects on pupil sizes following smoked, vaporized and oral cannabis administration.
Journal of applied toxicology : JAT, 37(8), 922-932
Citation
Newmeyer, Matthew N; Swortwood, Madeleine J; Taylor, Megan E; Abulseoud, Osama A; Woodward, Thomas H; Huestis, Marilyn A. (2017). Evaluation of divided attention psychophysical task performance and effects on pupil sizes following smoked, vaporized and oral cannabis administration.. Journal of applied toxicology : JAT, 37(8), 922-932. https://doi.org/10.1002/jat.3440