Drug molecules undergo changes to their intermolecular binding patterns under extreme conditions, leading to structural phase transitions which produce different polymorphs. Polymorphism of aspirin (acetylsalicylic acid), one of the most widely consumed medications, has attracted many scientists, chemists and pharmacologists to identify its stable polymorphs and phase transformations at ambient temperatures and pressures. Here, density functional theory at the ωB97XD/6-31G* functional level is utilized to calculate the lattice constants, volumes, Gibbs free energies, vibrational spectra, stabilities and phase transitions of aspirin forms I and II at different pressures and temperatures. These computations confirm that phase transformation occurs between these two forms of aspirin at higher pressures (from 3 to 5 GPa) and near room temperatures. Taking aspirin as a case study, this work can help design, produce and store drugs, guiding scientists, chemists and pharmacologists to perform further experiments.