Do Delta9-tetrahydrocannabinol concentrations indicate recent use in chronic cannabis users?
OBJECTIVE
To quantify blood Delta(9)-tetrahydrocannabinol (THC) concentrations in chronic cannabis users over 7 days of continuous monitored abstinence.


METHODS
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.


METHODS
Whole blood cannabinoid concentrations were determined by two-dimensional gas chromatography-mass spectrometry.


RESULTS
Nine chronic users (36%) had no measurable THC during 7 days of cannabis abstinence; 16 had at least one positive THC > or =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.
Karschner
Erin L
EL
Chemistry and Drug Metabolism, Intramural Research Program, National Institute on Drug Abuse, National Institutes of Health, Baltimore, MD, USA.
Schwilke
Eugene W
EW
Lowe
Ross H
RH
Darwin
W David
WD
Pope
Harrison G
HG
Herning
Ronald
R
Cadet
Jean L
JL
Huestis
Marilyn A
MA
D000328:Adult; D015415:Biomarkers / Q000097:blood; D013759:Dronabinol / Q000097:blood*; D005260:Female; D008401:Gas Chromatography-Mass Spectrometry; D006801:Humans; D008297:Male; D002189:Marijuana Abuse / Q000175:diagnosis*; D008875:Middle Aged; D011619:Psychotropic Drugs / Q000097:blood*; D015813:Substance Abuse Detection / Q000379:methods; D055815:Young Adult
D015415:Biomarkers; D011619:Psychotropic Drugs; D013759:Dronabinol