The developed method was validated according to ICH guidelines with respect to specificity, linearity, limits of detection, quantification, accuracy, precision, and robustness. The stability-indicating reverse-phase high-performance liquid chromatography (RP-HPLC) method is precise; it has been developed for the simultaneous estimation of assay of guaifenesin (GN) and dextromethorphan hydrobromic (HBr) (DN) in drug substance and drug product. The chromatographic separation was done in an isocratic mode using the Syncronus C8 (250 × 4.6 mm, 5 μ particle size) column with mobile phase containing a 10 mM ammonium acetate in water (modulated pH 4.30 with orthophosphoric acid) and acetonitrile in the ratio of 60:40 (% v/v) used for efficient chromatographic separation. The flow rate of the mobile phase was 1 mL/min with ambient column temperature and detection of wavelength at 279 nm; injection volume 10 μL was fixed for achieving good elution of eluents. The retention time for GN was found to 3.46 minutes and DN was found to 7.58 minutes. GN and DN were linear in the concentration range from 357 to 1,428 and 19 to 75 μg/mL, respectively. Regression analysis showed that the r value (correlation coefficient) greater than 0.999 for GN r value was found to be 0.999, GN r value was found to be 0.999, DN r value was found to be 0.999. Limit of detection (LoD) and limit of quantification (LoQ) of GN was found to be 0.151 and 0.904 μg/mL, DN was found to be 0.241 and 0.726 μg/mL. The developed method was validated and found to be accurate, specific, and robust. Both the drugs were subjected to the stress conditions like acidic, basic, oxidative, photolytic, and thermal conditions. The degradation results were found to be satisfactory. In peroxide stress condition, GN was found stable over DN, and DN was found to degrade significantly. The degradation products were well resolved from GN, DN, and their impurities. The peak purity test results confirmed that the GN and DN peak were homogenous and pure in all stress conditions, thus, proving the stability-indicating nature of the method. This method could be applied for the simultaneous estimation of GN and DN in drug substance and drug product.