In the present effort, a highly responsive electrochemical sensor made up of a carbon nanotube paste electrode modified with a thin layer of electrochemically polymerized L-Leucine was employed for the determination of cetirizine (CTZ) in the presence of paracetamol. In contrast to bare carbon nanotube paste electrode (BCNTPE), a noticeable shift in the peak potential along with an enhancement in the peak current was observed for the electro‒oxidation of CTZ on the modified electrode surface, which can be associated to the extended microscopic active surface area of the modified electrode. The surface of both unmodified and modified electrode surfaces were characterized using field emission scanning electron microscopy (FE-SEM) and electrochemical impedance spectroscopy (EIS) studies. The influence of experimental circumstances on the response of electrode, such as number of sweeping segments in the polymerization process, accumulation potential, accumulation time, and the pH were examined. Under the optimized experimental conditions, the calibration plot was achieved in the concentration range from 5 μM to 50 μM of CTZ with a regression co-efficient of 0.996. The evaluated detection limit for CTZ was achieved as 0.17 μM. The proposed electrode and method were effectively utilized to analyze CTZ in pharmaceutical, blood serum and urine sample with a recovery of 96.49‒99.85 %.