We report the high-pressure response of three forms (alpha, delta, and gamma) of pyrazinamide (C5H5N3O, PZA) by in situ Raman spectroscopy and synchrotron X-ray diffraction techniques with a pressure of about 14 GPa. These different forms are characterized by various intermolecular bonding schemes. High-pressure experimental results show that the gamma phase undergoes phase transition to the beta phase at a pressure of about 4 GPa, whereas the other two forms retain their original structures at a high pressure. We propose that the stabilities of the alpha and delta forms upon compression are due to the special dimer connection that these forms possess. On the other hand, the gamma form, which does not have this connection, prefers to transform to the closely related beta form when pressure is applied. The detailed mechanism of the phase transition together with the stability of the three polymorphs is discussed by taking molecular stacking into account.