<p>This research investigates the application of Expired Tilmicosin Drug as a corrosion inhibitor for C-steel in a 1&#xa0;M HCl solution. FT-IR measurements, atomic force microscopy (AFM), electrochemical impedance spectroscopy (EIS), weight loss (WL), and potentiodynamic polarization (PDP) were employed to evaluate the efficacy of Expired Tilmicosin Drug in protecting C-steel against corrosion. According to the findings, the inhibition efficiency (% IE) increased as the Expired Tilmicosin Drug concentration increased and reached 91.8% at 300 ppm, 25&#xa0;°C, but decreased to 85.6% at 45&#xa0;°C. The investigated drug acted as a mixed–kind inhibitor from the data of PDP technique. The Langmuir adsorption model was supported by the drug’s adsorption behavior on the C-steel surface. The adsorption phenomena was found to be spontaneous based on the computed values of the standard free energy change of adsorption (ΔG^o_ads). Fourier transform infrared spectroscopy (FT-IR) and atomic force microscopy (AFM) demonstrated that the drug molecules had a strong bond with the C-steel surface. Density functional theory (DFT) calculations and molecular dynamics (MC) simulations provided further insight into the chemical interactions between Expired Tilmicosin Drug and the C-steel surface. This study confirms that there is agreement between experimental and theoretical results. This research introduces the novel application of Expired Tilmicosin Drug, highlighting its non-toxic nature and cost-effectiveness, making it a promising alternative for corrosion prevention in industrial applications. This study investigates the dual role of Expired Tilmicosin Drug in addressing expired pharmaceutical waste and developing an efficient corrosion inhibitor for C-steel in acidic environments. Repurposing Expired Tilmicosin Drug provides a sustainable solution to environmental hazards while demonstrating high corrosion inhibition efficiency.</p>