The quantification of drugs of abuse in keratinized matrices is becoming of special relevance for monitoring consumption and for post-mortem investigations. We aimed to implement an analytical method for the simultaneous detection of morphine (MORF), 6-monoacetylmorphine (6-MAM), methadone (MET), 2-ethylidene-1,5-dimethyl-3,3-diphenylpyrrolidine (EDDP) and 2-ethyl-5-methyl-3,3-diphenylpyrrolidine (EMDP) in nails. After decontamination, the nail samples (30 mg) were submitted to an alkaline digestion followed by a two-step liquid-liquid and SPE extraction using mixed-mode cation exchange cartridges. The analytes were eluted with 5% NH4OH/methanol. After derivatization with N-methyl-N-(trimethylsilyl) trifluoroacetamide, the analytes were quantified by gas chromatography-mass spectrometry. The method was optimized and fully validated only for MET, EDDP and EMDP, since for MOR and 6-MAM it was not possible to obtain adequate recovery rates after extraction, although detection of MOR was still possible. The method was selective, accurate and precise. Regression analysis demonstrated linearity over a concentration range of 20.8-333.3 ng/mg for MET and 10.4-166.7 ng/mg for EDDP and EMDP. Limits of detection and quantification values ranged from 3.3 to 6.0 ng/mg and 10.4 to 20.8 ng/mg, respectively, and recovery rates ranged from 82% to 98%. The applicability of the method was demonstrated by analyzing nail and urine samples obtained from heroin consumers under substitution therapy with MET.