Can Hair Treatment Lower THC Without an Obvious Change in Appearance?
Hair can look substantially unchanged after a treatment that lowers its measured THC content. A laboratory study using authentic THC-positive hair found that visible appearance was an unreliable guide to whether the analytical concentration had changed.
What was treated and measured
Bertges and colleagues examined 54 THC-positive hair samples. Portions were exposed in vitro to five commercially available products, with untreated portions serving as comparisons. THC was quantified by liquid chromatography with tandem mass spectrometry.
The experiment concerned chemical changes to collected hair under specified laboratory conditions. It did not follow people using products over time, measure intoxication, or test whether a particular person had altered a specimen before collection.
A lower concentration need not mean less original exposure
The product groups showed mean THC reductions of approximately 52% to 65%. Fourteen of the 54 treated samples fell below the 0.02 ng/mg comparison threshold used in the study. Those figures describe this experiment; they are not a universal correction factor or a statement of the current legal meaning of a hair result.
The authors also reported a lack of obvious visible alteration. That finding weakens the assumption that only visibly damaged or conspicuously bleached hair could have experienced a relevant chemical change. It does not establish that any normal-looking negative specimen has been manipulated.
What a forensic review should establish
The relevant history includes cosmetic treatment, the segment examined, sample condition and the laboratory’s preparation and confirmation procedures. Interpretation should distinguish an altered analyte concentration from a demonstrated change in the person’s original cannabis exposure.
There were unequal and sometimes small numbers of samples across product groups. Individual hair characteristics and the specific experimental exposure also limit generalization. A court or clinician cannot reconstruct the original THC concentration by simply doubling the measured value or applying the study’s average percentage loss.
The broader uncertainty is discussed in our article on why hair THC does not reliably reveal the quantity of cannabis consumed. Treatment history adds another reason to avoid turning one hair concentration into an exact consumption estimate, an abstinence guarantee or a measure of impairment at a particular time.
Primary research
Manipulation of THC Hair Concentrations by Commercially Available Products. DOI: 10.3390/metabo12100900.
