Crystalline solids are characterized by long-range positional and orientational order in three dimensions, whereas amorphous liquids lack long-range order in any dimension. Liquid crystals (mesophases) show structural, mechanical and optical properties intermediate to those of crystalline solids and the amorphous, liquid state of matter. There are two principle types of liquid crystals: thermotropic liquid crystals (TLCs) and lyotropic liquid crystals (LLCs). TLCs can be formed by heating a crystalline solid or by cooling an isotropic melt of a TLC-forming molecule (mesogen). In the first part of this review the types of liquid crystals are defined and classified and the structural properties of mesogens are explained. In the second part, ten case studies of thermotropic mesomorphous drugs and pharmaceutically relevant molecules (arsphenamine, nafoxidine hydrochloride, L-660711, palmitoyl propranolol hydrochloride, penbutolol sulfate, itraconazole hydrochloride, fenoprofen sodium, fenoprofen calcium, ciclosporin and cholesteryl esters) are presented and their thermotropic mesomorphism is described. The review closes with a brief discussion of the unusual properties of drug mesophases and a potential use of drugs and excipients in this fourth state of matter.
