By Paul Armentano October 2009
Trace levels of delta-9-THC, the primary psychoactive ingredient in marijuana, may be identifiable in the blood of heavy cannabis users for up to seven days after their drug use has ceased, according to clinical trial data to be published in the journal Addiction.
Investigators at the National Institutes of Health in Baltimore quantified blood THC concentrations in 25 chronic cannabis users over a period of seven days of continuous monitored abstinence.
Researchers reported, “On day seven, six full days after entering the unit, six participants still displayed detectable THC (blood) concentrations. … The highest observed THC concentrations on admission (day 1) and day seven were 7.0 and 3.0 ng/ml, respectively.”
They concluded, “Substantial whole blood THC concentrations persist multiple days after drug discontinuation in heavy chronic cannabis users.”
Previous studies had indicated that low levels of THC typically remain present in the blood of chronic users for a period of time not exceeding 48 hours.
Investigators noted that all of the subjects who tested positive for the presence of THC in their blood on day seven were female. Body mass index did not correlate with time until the last THC-positive specimen.
Researchers also noted that several subjects continued to test positive for THC in their blood even after initially testing negative for it on day one.
Commenting on the study, NORML Deputy Director Paul Armentano said, “These findings challenge the conventional wisdom that the detection of low levels of THC in blood are necessarily associated with recent cannabis use or impairment. In particular, these findings call into question the use of per se DUI drug driving laws, which presently treat drivers with any levels of THC in their blood as criminally impaired. These results indicate, once again, that drug testing results possess limited value when making judgments regarding marijuana use or impairment.”
Full text of the study, “Do Delta(9)-tetrahydrocannabinol concentrations indicate recent use in chronic cannabis users,” will appear in Addiction.