Abstract A simple voltammetric method based on a composite biosensor MWCN/SiO2/Al2O3/Nb2O5/DNA (MWCN/SiAlNb/DNA) was developed and validated for the determination of promethazine (PHZ) in raw material and pharmaceutical formulation. The best results were obtained by using square wave voltammetry under measurements at 0.30molL−1 phosphate buffer pH 7.0 and DNA immobilization onto SiAlNb surface using 0.8mgmL−1 concentration. Validation parameters such as: selectivity, linearity, precision, accuracy, and stability presented results within the acceptable range. The sensitivity of sensor was found to be 0.210μAμmol−1L. Therefore, the developed method was successfully applied for the PHZ determination in real samples of pharmaceutical formulations and can be used for routine quality control analysis of bulk pharmaceutical and pharmaceutical formulations containing PHZ.