Converting two poorly water-soluble crystalline drugs to co-amorphous drug systems by ball milling, quench-cooling, or cryo-milling method can improve stability of the drug, enhance dissolution rates, and reduce adverse reactions of the single drug.  Co-amorphous system has been used to solve problems of co-administration of medicines.  Formation and intermolecular interactions of co-amorphous drug systems may be verified by differential scanning calorimetry (DSC), X-ray powder diffraction (XRPD), raman spectroscopy (RS) and fourier transform infrared spectroscopy (FT-IR).  Stability of co-amorphous drug systems is influenced by their glass transition temperature (Tg) and intermolecular interactions.  The theoretical Tg values and the interaction parameter x are calculated by Gordon-Taylor equation and the Flory-Huggins equation, respectively.  Thus, co-amorphous drug systems are analyzed theoretically at molecular level.  Co-amorphous drug systems provide a new sight for the co-administration of medicines.