Marijuana Hair Testing: An Accurate Method for Detecting Consumption?

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.