Do Δ9-Tetrahydrocannabinol Concentrations Indicate Recent Use in Chronic Cannabis Users?

AIMS—To quantify blood Δ9-tetrahydrocannabinol (THC) concentrations in chronic cannabis users over seven days of continuous monitored abstinence. PARTICIPANTS—Twenty-five frequent, long-term cannabis users resided on a secure clinical research unit at the U.S. National Institute on Drug Abuse under continuous medical surveillance to prevent cannabis self-administration. MEASUREMENTS—Whole blood cannabinoid concentrations were determined by two-dimensional gas chromatography-mass spectrometry. FINDINGS—Nine chronic users (36%) had no measurable THC during seven days of cannabis abstinence; 16 had at least one positive THC ≥0.25 ng/mL, but not necessarily on the first day. On day 7, six full days after entering the unit, six participants still displayed detectable THC concentrations (mean ± SD, 0.3 ± 0.7 ng/mL) and all 25 had measurable carboxy-metabolite (6.2 ± 8.8 ng/mL). The highest observed THC concentrations on admission (day 1) and day 7 were 7.0 and 3.0 ng/mL, respectively. Interestingly, five participants, all female, had THC positive whole blood specimens all seven days. Body mass index did not correlate with time until last TH