The unique molecular structure of Closantel, a pharmaceutical compound, with a high number of heteroatoms, aromatic structures, and multiple bonds can introduce the expired form of the pharmaceutical compound as a potent anti-corrosion agent (ACA) for carbon steel in acidic solutions. In this way, the present study explored the employment potential of the expired pharmaceutical compound as a sustainable ACA using experimental and computational approaches. The results demonstrated that the chemi/physisorption (ΔG_ads ≈ −20.5 kJ/mol) of the expired pharmaceutical compound on the soaked metal hindered the access of aggressive agents, with a significant inhibition efficiency (up to 95.5 %) at the optimum dosage (800 ppm).