As the study focused on DXM stability study, only degradation products i.e., Imp.A and Imp.C were considered with a limit of 0.5% in the pharmaceutical product. Imp.B and Imp.D are not degradation products but synthesis by-products, therefore the setwo impurities are not expected to increase during stability stud-ies. Consequently, they were not included in method development.The structures of DXM and its related substances are presented in Fig. 1.