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Amitriptyline hydrochloride is an antidepressant drug with sedative effects used to treat the symptoms of anxiety, agitation with depression and schizophrenia with depression. A reversed‐phase high‐performance liquid chromatography method was developed to separate and quantitatively determine the assay and four organic impurities of amitriptyline in tablet dosage form and bulk drugs using a C18 column in an isocratic elution mode with mobile phase consisting of a mixture of pH 7.5 phosphate buffer and methanol. The pH conditions used in the chromatographic separation are discussed. The stability‐indicating characteristics of the proposed method were proved using stress testing [5 m HCl at 80°C/1 h, 5 m NaOH at 80°C/1 h, H2O (v/w) at 80°C/1 h, 6% H2O2 (v/v) at 25°C/1 h, dry heat at 105°C/24 h and UV–vis light/4 days] and validated for specificity, detection limit, quantitation limit, linearity, precision, accuracy and robustness. For amitriptyline and its four known organic impurities, the quantitation limits, linearity and recoveries were in the ranges 0.25–3.0 μg/ml (r2 > 0.999) and 87.9–107.6%, respectively. The mass (m/z) spectral data of amitriptyline hydrochloride and its impurity are discussed. The proposed LC method is also suitable for impurity profiling and assay determination of amitriptyline in bulk drugs and pharmaceutical formulations. [ABSTRACT FROM AUTHOR]