Chemical stability of amitriptyline hydrochloride in oral liquid dosage forms

The chemical stability of amitriptyline hydrochloride in two oral liquid dosage forms has been studied using a stability-indicating high-performance liquid chromatographic assay method that was developed in our laboratory. The concentrations of the drug were directly related to peak heights, with an R value of 0.999. The percent relative standard deviation based on five injections was 1.0. The products of decomposition and excipients present in the dosage forms did not interfere with the developed assay method. The formulation which contained edetate disodium was stable for at least 37 days when stored in amber-colored glass bottles at room temperature. However, the pH value of this formulation decreased from 5.0 to 4.0 after 37 days of storage. The formulation without edetate disodium was stable for only 21 days (borderline) under the same storage conditions. The pH value of this formulation remained constant at 5 after 37 days of storage at room temperature and the physical appearance of both formulations did not change.