Topological rules for constructing (p,T) diagrams of drug polymorphs are discussed following a classical approach founded upon the Ostwald's principle of least vapor pressure. This method allows the definition of hierarchy stabilities among the crystal varieties of a polymorph as a function of temperature and pressure, it has been applied to two trimorphic drugs. sulfanilamide and piracetam. The overall monotropy of the metastable orthorhombic alpha-variety of sulfanilamide was demonstrated. in fact, it cannot possess any stability region, even at high pressures. The method for piracetam showed a sensitivity to the set of experimented data to which it is applied. Depending on the values of the transition temperatures of forms II (triclinic) and III (monoclinic) to form I(monoclinic), three kinds of (p.T) representations were derived. Preliminary applications of a high-pressure piezothermal method did not discern the three different kinds of (p.T) diagrams.