The forensic limits of hair drug testing: a critical assessment of drug intake and hair concentration relationships

• Hair analysis confirms exposure but cannot quantify drug dose. • Dose–concentration in hair is inconsistent across drug classes. • Melanin, sweat, and cosmetic treatments confound hair drug levels. • Controlled studies show trends but are limited by ethical constraints. • Hair should be used qualitatively, not for dose estimation in forensics. Hair analysis is widely used in forensic toxicology to establish drug exposure, offering a long detection window and non-invasive sampling. This review provides a critical assessment of whether a dose-concentration relationship between drug intake and concentrations measured in hair exists and if such a relationship would be suitable for forensic purposes. A systematic search as conducted to identify controlled studies that were published across illicit substances and pharmaceuticals. The results reveal substantial variability in reported correlations. While some pharmaceuticals (e.g., clozapine, methadone) show moderate associations, most illicit drugs, including heroin, codeine, amphetamine, cannabis, and GHB, show weak or inconsistent relationships. Overlapping concentration ranges between low and high doses, combined with confounding factors such as melanin content, sweat contamination, cosmetic treatments, and methodological inconsistencies, undermine the reliability of dose estimations. Although controlled administration studies provide more robust data, ethical constraints limit their scope. Current evidence indicates that hair analysis can confirm exposure but cannot accurately quantify dose or distinguish intentional ingestion from passive contact. Future research should focus on standardizing analytical protocols, correcting for melanin content, and exploring pharmaceuticals with predictable dosing patterns. Overall, hair should be regarded as a qualitative indicator of exposure rather than a quantitative measure of drug intake.