Dinko Kranjac, PhD, Medical Editor
In 2015, the researchers at the Institute of Forensic Medicine in
Freiburg, Germany showed that the presence of cannabinoids in an
individual's hair does not prove that the person consumed marijuana.
More specifically, their findings, published in Scientific Reports,
indicated that tetrahydrocannabinol (THC), the main active chemical
ingredient found in cannabis, can be found in hair of non-consuming
individuals due to transfer through cannabis consumers via their hands
or sweat, or through passive marijuana-smoke exposure.
While hair testing is widely used in employee drug screening and in
child protective cases, their findings contradicted the prevailing view
that hair testing is an accurate method for testing cannabis
consumption.
In this study, one male study participant ingested 50 mg THCA-A
(tetrahydrocannabinolic acid A), the biogenetic precursor of THC, once
per day over a 30-day period. Head, chest, pubic, axillary, and leg hair
samples were collected before, as well as once weekly for 3 weeks
after, the last intake.
Two different male participants orally ingested
2.5 mg THC 3 times per day over a 30-day period. Head, beard, and body
hair samples were collected prior to, as well as once weekly for several
weeks after, the last intake.
Although the participant ingested a “relatively high dose of THCA-A,”
the results strongly suggest that THCA-A is not incorporated into hair
through the bloodstream or via sebum/sweat to a relevant extent,” the
investigators noted. Similarly, repeated oral intake of THC did not lead
to THC detection in any of the collected hair samples.
Thus, “oral
uptake of THC or cannabis products does not necessarily lead to positive
THC hair findings, which can be of interest in abstinence control,” the
German researchers noted.
Since THC or THC metabolites are not incorporated from bloodstream
into hair to a significant extent, “THC detected in forensic hair
samples does originate from external sources,” they concluded.
New Findings
New findings published in Drug and Alcohol Review, however, indicate that cannabinoids can be detected in hair samples. More specifically, THC was detected in 77% and 39% of heavy and light cannabis users, respectively.
In this study, 105 hair samples were included in the analysis, and
25% and 36% of the participants were heavy and light users,
respectively. The sensitivity of THC detection is higher when compared
with that of other metabolites such as THC-OH, THC-COOH, cannabinol, and
cannabidiol, the authors noted.
In contrast to the findings of a study published previously in Scientific Reports,1
the investigators affiliated with the University of Bristol, Imperial
College London, and Oxford University did not detect any THC or THC-COOH
in non-users.
Thus, according to the present report, hair analysis is an appropriate method for detecting cannabis
consumption within the past 3 months. However, “this approach is unable
to reliably detect light cannabis consumption or determine the quantity
of cannabis used by the individual,” British scientists concluded in
their publication.
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