An expired drug with non-toxic characteristics has been evaluated as a corrosion inhibitor for mild steel alloy. The corrosion inhibition efficiency of the expired ethambutol drug in a 0.5 M HCl solution has been studied using weight loss, electrochemical impedance spectroscopy (EIS), electrochemical polarization, scanning electron microscopy (SEM) and molecular dynamics (MD) techniques. The results showed that the drug provides appreciable inhibition efficiency, more than 95% at the higher concentration, i.e., 1000 ppm. The Tafel polarization plots have shown that the expired drug acted as a mixed type inhibitor. Langmuir adsorption isotherm, along with physiochemical mode of interaction, has proved that the corrosion inhibition process accords with the isotherm. The outcomes obtained from all the experimental techniques and theoretically obtained information are in good correlation. MD simulations reveal that the studied compound adsorbs onto the surface of mild steel in the planar orientation. © 2018, Sociedade Portuguesa de Electroquimica. All rights reserved.