Morning-after THC tests may not reliably prove driving impairment

Some jurisdictions use blood delta-9-tetrahydrocannabinol (THC) thresholds of 2ng/mL and oral fluid THC thresholds of 25ng/mL for detection of impaired driving. This study assesses morning-after driving and cognitive performance following evening cannabis use in participants above or below the 2ng/mL blood and 25ng/mL oral-fluid THC thresholds. This observational study included frequent cannabis users (≥ 4 times/week) who smoked their preferred cannabis at home the evening before laboratory testing. At the visit (12-15 h post-use), participants completed driving simulation trials and underwent cognitive testing (verbal free recall and trail making test). Blood and oral fluid samples were collected at the time of testing for cannabinoid quantification. Outcomes were compared between participants above versus below the blood and oral fluid THC cut-offs, with correction for multiple comparisons. Sixty-five frequent users participated. For driving outcomes, participants with blood THC ≥ 2 ng/mL showed greater variability in following distance than those below the cut-off (Cohen's d = - 0.58), and those with oral fluid THC ≥ 25 ng/mL demonstrated slower reaction time (Cohen's d = - 0.85); however, neither effect remained statistically significant after correction for multiple comparisons. Participants with oral fluid THC ≥ 25 ng/mL performed significantly worse on Trail Making Test A and B, with large effect sizes (Cohen's d > 1.0) that remained significant after corrections, and showed worse delayed verbal free recall with medium-to-large effect sizes, which did not remain statistically significant after corrections. Blood THC cut-offs were associated with medium effect sizes on cognitive measures, but these did not remain statistically significant after correction. Per-se THC cut-offs showed subtle associations with simulated driving performance, but clearer associations with changes in cognitive performance, particularly for oral fluid THC. These findings highlight the limitations of relying solely on biological THC thresholds to infer impairment during the residual phase.