The inhibiting effect of diphenoxylate drug (DD) on API X120 carbon steel corrosion in 15% HCl solution was investigated using chemical, electrochemical and surface morphological studies. The inhibition efficiency of the studied drug increases with increase in its concentrations, giving a maximum inhibition efficiency of 95.28%, at the optimum concentration of 400 ppm. The effect of temperature revealed that the inhibition efficiency of DD decreases with temperature rise. The adsorption of DD obeyed the Langmuir isotherm and indicated predominantly physical adsorption mechanism. The electrochemical impedance spectroscopy (EIS) analysis affirmed the adsorption of the inhibitor on the X120 steel surface. Potentiodynamic polarization (PDP) study indicated that the tested drug acted as mixed type inhibitor with little anodic dominance. The result of scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) spectroscopy supported the formation of adsorbed inhibitor film on the X120 steel surface.