Inhibition performance and adsorption behavior of Expired Pharmaceutical drugs on metals were studied by quantum chemical calculations, this method very useful in determining molecular structure and to study compounds reactivity. Expired Pharmaceutical drugs corrosion inhibition properties determine by using MOPAC 2016 with Parameterized Model 3 (PM3), Quantum chemical calculations were down. Quantum chemical methods nowadays have become common practice to carry out virtual determinations in corrosion inhibition studies. Quantum chemical parameters such as energy levels of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), energy gap(ΔE), global hardness (η), global s softness (S), absolute electro negativity (χ), and electrophilicity index (ω) electronic density is virtually identified. Structure and activity of compounds can be readily screened, employing computational methodology with help of set of mathematical equations that are capable of representing accurately the chemical phenomenon under study. Present study elucidates inhibition efficiency of Expired Pharmaceutical drugs in acidic media. This research was performed using quantum chemical calculations by means of MOPAC 2016 with PM3 method to find correlation between molecular structures of compounds and their behavior as corrosion inhibitors. Results showed that Expired Pharmaceutical drugs (Ofloxacin, Ciprofloxacin, Norfloxacin and Gentamicin) inhibit efficient and effective corrosion of mild steel in hydrochloric acid media.