The Effect of Permanent Dye Colour on Illicit Drug and Metabolite Levels in Chemically Treated Hair.

Hair analysis for drugs of abuse is widely applied in forensic contexts, particularly in family law cases where child protection is a critical consideration. It is recognised that adulteration of hair samples via chemical treatments can occur; however, the specific influences of dye colour on parent drug and metabolite levels have yet to be systematically examined. In this study, 100 authentic hair samples positive for tetrahydrocannabinol (THC), 11-nor-9-carboxy-Δ9-tetrahydrocannabinol (THC-COOH), cocaine (COC) and benzoylecgonine (BZE) were treated with nine permanent dyes and one bleach product. Concentrations were quantified using validated LC-MS/MS methods, and original and post-treatment values were compared statistically via the two-tailed matched pairs t-test. Results demonstrated no consistent influence from dye colour on analyte concentrations. Following treatment, THC and COC concentrations predominantly decreased, consistent with oxidative degradation and reduced binding to the hair matrix. In contrast, THC-COOH and BZE concentrations increased in the majority of samples, suggesting in situ formation from peroxide-mediated oxidation of parent compounds, as well as increased residue release from damaged keratin structures. Bleaching produced the most pronounced changes, with significant decreases in THC, THC-COOH and COC levels and significant increases in BZE. COC was the analyte least affected by treatments overall. These findings indicate that dye colour itself is irrelevant, although chemical treatments can significantly impact toxicological results. Broader implications, such as increased risks of misinterpretation of results from artificially increased or decreased levels (shifted above or below reporting thresholds) and subsequent inaccurate conclusions regarding drug use, must be considered when analysing chemically treated hair samples.