Density functional theory DFT (B3LYP)/6-31G (d) has been applied to compute the geometry optimization, dipole moment and vibrational frequencies of aspirin, paracetamol and phenacetin. From the comparison of vibrational frequencies, it is observed that the value of O-H stretching in aspirin is lower as compared to paracetamol shows its greater tendency to form cation whereas paracetamol has least tendency to form cation as the O-H stretching in paracetamol is at greater frequency even than that of phenacetin, which explains that why there is displacement of –COOH by –OH group in aspirin i.e. (aspirin) acetyl-salicylic acid readily converts into salicylic acid, which is responsible for most of its actions. The greater values of O-H and C-H stretching of paracetamol than aspirin and phenacetin explains its selective behavior to inhibit cyclooxynase-2 (COX-2) and poor inhibitor of the synthesis of prostaglandins, which is responsible for pain headache and infection. On comparison of dipole moment of it is also observed that the order is Aspirin > Paracetamol > Phenacetin and from our earlier ADMP studies of total energy curve vs time in trajectory curve of aspirin, paracetamol and pehnacetin at DFT/6-31G(d) it is found that the energy required to release Hydrogen from these drugs are in the order Paracetamol > Phenacetin > Aspirin, therefore from infrared spectra, dipole moment and ADMP studies of aspirin, paracetamol and phenacetin it is concluded that aspirin is most reactive which explains that why it inhibit the synthesis of prostaglandins responsible for pain, headache and fever; restricts the blood supply to the tumor in effectnot allowing it to grow than a pea and paracetamol is a poor inhibitor of prostaglandins synthesis, does not have antiinflammatory action; phenacetin shows more reactive sites than paracetamol but less than aspirin, which give the evidence that why it is being replaced aspirin.