A sensitive spectrophotometric method for the analysis of isoniazid, lisinopril dihydrate, amoxicillin trihydrate, ampicillin trihydrate, glucosamine sulfate, phenylpropanolamine hydrochloride and gabapentin is described. The analysis is based on the reaction of drug molecules with ninhydrin and sodium molybdate mixture to give Ruhemann’s purple product with maximum absorbance (λmax) at 570 nm. The statistical analysis of intra-day and inter-day estimation of drugs as well as comparison with reported methods demonstrated high precision and accuracy of the proposed method. The method was successfully applied to the analysis of pharmaceutical preparations. The procedure was suitable for quality control application.