A simple, precise and accurate isocratic RP-HPLC stability-indicating assay method has been developed to determine trihexyphenidyl and trifluoperazine in their combined dosage form. Isocratic separation was achieved on a Kromasil -C18, column at room temperature in isocratic mode, the mobile phase consists of Methanol: Sodium Acetate in water (80:20, v/v) at a flow rate of 1.0 ml/min, the injection volume was 20μl and UV detection was carried out at 232nm.The drug was subjected to acid and alkali hydrolysis, oxidation, photolysis, UV light and heat as stress conditions. The method was validated for specificity, linearity, precision, accuracy, robustness and system suitability. The method was linear in the drug concentration range of 2–12 μg/ml and 5-30 μg/ml for trihexyphenidyl and trifluoperazine respectively. The precision (RSD) of six samples was 0.868 and 0.1.191% for repeatability, and the intermediate precision (RSD) among six-sample preparation was 1.212 and 0.803% for trihexyphenidyl and trifluoperazine, respectively. The mean recoveries were between 98.19–101.38% and 98.76–100.50% for trihexyphenidyl and trifluoperazine respectively. The proposed method can be used successfully for routine analysis of the drug in bulk and combined pharmaceutical dosage forms.
