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Aims: To assess the dose-effect relationship between self-reported drug intake and the concentration of drugs and/or their metabolites in hair and to examine factors that may mediate this relationship., Design and Setting: A cohort study among young drug users (YDU) in Amsterdam, the Netherlands, which began in July 2000. At intake, YDU were asked to report their average drug intake over a 2-month period. A hair sample was taken and then analysed for cocaine, benzoylecgonine (BE), morphine, 6-monoacetylmorphine and methadone. Weighted least-squares regression analysis was used to model hair-test results as a function of reported drug use., Participants: Subjects were 95 YDU (using cocaine, heroin, methadone and/or amphetamines at least 3 days/week) aged 18-30 residing in Amsterdam in 2000-2001., Findings: Of the 95 YDU, one-third were women; mean age was almost 26; 30% had black hair, 33% blond hair and 37% brown hair. Cocaine use was reported by 92%, heroin by 75% and methadone by 64% of participants. All hair samples contained one or more drugs. Crude correlation coefficients between reported drug doses and drug concentrations in hair ranged between 0.45 and 0.59. The multivariate regression analysis showed that, for one or more types of drug, black-haired people, women and non-western European people had relatively high drug concentrations in hair (significant slope effects). The corresponding multivariate correlation coefficients ranged between 0.63 and 0.87., Conclusions: Hair testing can be used to quantify drug use in epidemiological studies, given that factors such as hair colour and sex are taken into account.