Do Δ9-tetrahydrocannabinol concentrations indicate recent use in chronic cannabis users?

Aims  To quantify blood Δ9-tetrahydrocannabinol (THC) concentrations in chronic cannabis users over 7 days of continuous monitored abstinence. Participants  Twenty-five frequent, long-term cannabis users resided on a secure clinical research unit at the US 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 7 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, 6 full days after entering the unit, six participants still displayed detectable THC concentrations [mean ± standard deviation (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 over all 7 days. Body mass index did not correlate with time until the last THC-positive specimen (n = 16; r = −0.2; P = 0.445). Conclusions  Substantial whole blood THC concentrations persist multiple days after drug discontinuation in heavy chronic cannabis users. It is currently unknown whether neurocognitive impairment occurs with low blood THC concentrations, and whether return to normal performance, as documented previously following extended cannabis abstinence, is accompanied by the removal of residual THC in brain. These findings also may impact on the implementation of per se limits in driving under the influence of drugs legislation.