Lipid excipients are abundantly distributed in nature, biodegradable and are generally regarded as safe (GRAS). It enhances the absorption of lipophilic drugs and therefore, has evoked a boost in application of lipids as carrier for most lipophilic drugs. Lipid based drug delivery systems (LBDDS) have feasibility of different delivery routes including oral, parenteral, pulmonary, topical, ocular, intravaginal and intranasal among others. LBDDS may be designed to target drug to a particular region of the body or as controlled drug delivery for different purposes. However, lipid as an excipient undergoes polymorphism just like some other materials. There is need for proper characterisation of particular lipid to be used for drug delivery as well as proper preformulation studies. The molecular structures of lipids play distinct roles in the determination of its phase behavior. The packing ratio is often employed in order to determine the structure of formed crystalline phase. Lipid excipients are influenced by so many factors that may affect the physicochemical properties of lipid based formulations (LBFs) such as the liquid crystalline phase transition, lipid crystallization and polymorphism. This review is centered on the physicochemical properties that influence the properties of lipids. Different polymorphic properties of lipids would be reviewed as well as the applications of different polymorphic forms in drug delivery. Also, phase transition temperature (Tm) of some lipids as well as factors that influence Tm would be discussed. This work would also discuss some guidelines in formulating LBFs, factors that affect the bioavailability of ingested drugs as well as recent advances in LBDDS.

Keywords: Lipid crystallization, polymorphism, hexososmes, cubosomes, micelles, liposomes