Pharmacokinetics of Disappearance of Cocaine from Hair After Discontinuation of Drug Use

Unlabelled: Methods that employ detection of drugs of abuse in hair are important for monitoring compliance with drug abstinence. Understanding the mechanisms and timeline of drug disappearance from hair is critical for clinical and forensic application of hair testing. We aimed to evaluate the kinetics of disappearance of cocaine and its metabolite, benzoylecgonine (BE), from hair after discontinuation of drug use.

Methods: The Motherisk laboratory at the Hospital for Sick Children in Toronto routinely receives hair samples for toxicology analysis. Cocaine and BE hair results were obtained from the Motherisk Database for calculation of half-life of these compounds in hair. Subjects were included in the study if they had gradually decreasing concentrations of cocaine and/or BE in sequential hair samples, with higher levels in the 1-3 cm distal segments (i.e. earlier in time) and low or non-measurable levels in the segment closest to the scalp (i.e. closer to the date of sampling). Elimination half-life of cocaine and BE in hair was calculated using standard kinetics calculations. The study was anonymous, and received ethics approval by the Ethics Review Board of our institution.

Results: 137 subjects met the inclusion criteria for the study. The median half-life of cocaine in hair was 1.5 months (95% CI 1.2-1.8) in females and 1.5 months (95% CI 1.1-1.8) in males. The median half-life of BE was 1.5 months (95% CI 1.1-2) in females and 1.5 months (95% CI 0.8-1.8) in males. Half lives of cocaine or BE were not statistically different between males and females (Mann-Whitney U-test; P=0.93 for cocaine, P=0.99 for BE). Half lives of cocaine and BE were strongly correlated (Spearman rank rho=0.73; P<0.001).

Conclusion: Cocaine and BE could be detected in hair of former drug users for several months after abstinence. The calculated half-life of over 1 month for cocaine implies that, assuming first order elimination, approximately 3-4 months have to pass for hair testing to become negative in the segment proximal to the scalp. This finding should be incorporated in interpreting compliance with abstinence of former drug users, and suggests that caution has to be exerted when evaluating potential breaches of abstinence.