A procedure is presented for the simultaneous identification and quantification of morphine (MOR), codeine (COD), ethylmorphine (EM), 6-monoacetylmorphine (6-MAM), cocaine (COC), benzoylecgonine (BZE), ecgonine methylester (EME) and cocaethylene (CE), contained in the hair of opiates and cocaine addicts. The method involves decontamination in dichloromethane, pulverization in a ball mill, heat-acid hydrolysis, addition of deuterated internal standards, liquid-liquid extraction and gas chromatography/mass spectrometry (GC/MS) after silylation. The limit of detection (LOD) was approximately 0.1-0.8 ng/mg for each drug, using a 30-mg hair sample. The method is reproductible, with a coefficient of variation (CV) of approximately 8-17%. Cocaine and 6-monoacetylmorphine were the major compounds detected in cases of cocaine (14 cases) and heroin (68 cases) intake. Concentrations were in the range 0.4-78.4 ng/mg (COC), 0.0-36.3 ng/mg (BZE), 0.0-1.6 ng/mg (EME), 0.0-2.1 ng/mg (CE), 0.0-84.3 ng/mg (6-MAM), 0.2-27.1 ng/mg (MOR) and 0.1-19.6 ng/mg (COD). An application in forensic sciences, involving multi-sectional analysis, is given.