A multi-walled carbon nanotube modified glassy carbon electrode (MWCNTs/GCE) has been used for simultaneous determination of acetaminophen (ACT) and codeine (COD) in phosphate buffer (pH 7) solution. The measurements were carried out using differential pulse voltammetry (DPV), cyclic voltammetry (CV) and chronoamperometry (CA) methods. CA measurements showed a wide linear range for determination of ACT from 5 to 400 μmol L−1, and a detection limit of 0.19 μmol L−1 (S/N = 3). For COD the linear range from 5 to 240 μmol L−1, and the detection limit of 0.20 μmol L−1, were obtained. The fabricated sensor revealed some advantages such as convenient preparation, high sensitivity, good repeatability and stability. The analytical performance of this sensor has been evaluated for detection of ACT and COD in human serum, human urine and one pharmaceutical preparation with satisfactory results.