Abstract <p>A sensitive and simple indirect spectrophotometric method has been developed for determining phenylephrine hydrochloride and isoniazid in both their pure forms and pharmaceutical formulations. The method relies on the oxidation of these drugs using a known quantity of N-bromosuccinimide in hydrochloric acid. The remaining oxidant then decolorizes Ponceau 4R dye, and the absorbance of the dye is measured at 507 nm. The method follows Beer’s law over concentration ranges of 5–18 and 6–18 μg/mL for phenylephrine hydrochloride and isoniazid, respectively, with molar absorptivity values of 2.2 × 10⁴ and 1.6 × 10⁴ L/(mol cm), and the average percentage recoveries were 99.10 and 100.61%, respectively. The relative standard deviation was below 1.38% for both drugs. The method was effectively applied to analyze pharmaceutical products from various sources, yielding results consistent with the labeled drug content and the standard addition method, confirming the absence of interference from excipients.</p>