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BACKGROUND In driving-under-the-influence cases, blood typically is collected approximately 1.5–4 h after an incident, with unknown last intake time. This complicates blood Δ9-tetrahydrocannabinol (THC) interpretation, owing to rapidly decreasing concentrations immediately after inhalation. We evaluated how decreases in blood THC concentration before collection may affect interpretation of toxicological results. METHODS Adult cannabis smokers (≥1×/3 months, ≤3 days/week) drank placebo or low-dose alcohol (approximately 0.065% peak breath alcohol concentration) 10 min before inhaling 500 mg placebo, 2.9%, or 6.7% vaporized THC (within-individuals), then took simulated drives 0.5–1.3 h postdose. Blood THC concentrations were determined before and up to 8.3 h postdose (limit of quantification 1 μg/L). RESULTS In 18 participants, observed Cmax (at 0.17 h) for active (2.9 or 6.7% THC) cannabis were [median (range)] 38.2 μg/L (11.4–137) without alcohol and 47.9 μg/L (13.0–210) with alcohol. THC Cmax concentration decreased 73.5% (3.3%–89.5%) without alcohol and 75.1% (11.5%–85.4%) with alcohol in the first half-hour after active cannabis and 90.3% (76.1%–100%) and 91.3% (53.8%–97.0%), respectively, by 1.4 h postdose. When residual THC (from previous self-administration) was present, concentrations rapidly decreased to preinhalation baselines and fluctuated around them. During-drive THC concentrations previously associated with impairment (≥8.2 μg/L) decreased to median &amp;lt;5 μg/L by 3.3 h postdose and &amp;lt;2 μg/L by 4.8 h postdose; only 1 participant had THC ≥5 μg/L after 3.3 h. CONCLUSIONS Forensic blood THC concentrations may be lower than common per se cutoffs despite greatly exceeding them while driving. Concentrations during driving cannot be back-extrapolated because of unknown time after intake and interindividual variability in rates of decrease.

Effect of Blood Collection Time on Measured Δ9-Tetrahydrocannabinol Concentrations: Implications for Driving Interpretation and Drug Policy