The incorporation rate of drugs into hair is strongly correlated with melanin affinity and, to a lesser extent, lipophilicity. Among 20 tested drugs, cocaine had the highest incorporation rate, while THCA had the lowest—a 3600-fold difference. Melanin affinity alone correlated with incorporation rate at r = 0.947; combining melanin affinity and lipophilicity raised the correlation to 0.979. Basic drugs showed higher incorporation rates than neutral or acidic ones, likely due to membrane permeability driven by the pH gradient between blood (pH 7.4) and the acidic hair matrix.
Lysergic acid diethylamide (LSD) and its metabolite norLSD can be detected in hair after administration. In pigmented rats given daily doses of 0.05 to 2 mg/kg for 10 days, LSD was found in newly grown hair even at the lowest dose, while norLSD appeared only at the highest dose. Analysis of hair from 17 self-reported LSD users detected LSD in two samples. The methods used gas chromatography-mass spectrometry and high-performance liquid chromatography with fluorometric detection, demonstrating the feasibility of hair analysis for LSD exposure.
In rats given acute poisonous doses of MDMA (20–100 mg/kg), the drug appeared in hair roots within minutes and reached high concentrations (up to 156 ng/mg). In surviving animals, MDMA levels in hair roots rose for up to 6 hours and then slowly declined over 24 hours. Washing removed more drug early on, indicating that MDMA is not immediately immobilized in hair roots but gradually becomes incorporated into the hair shaft. After death, MDMA concentrations stopped increasing, and the ratio of its metabolite MDA to MDMA plateaued, reflecting halted hair growth and metabolism. MDMA was incorporated into and retained in hair more quickly and firmly than methamphetamine.