Abstract: Ionic liquids (ILs) are liquid mixtures formed by anions and cations in a certain stoichiometric ratio under certain conditions. They are widely used in various fields because of their simple preparation process, low volatility, high stability, high conductivity and non-flammability. Here, we firstly introduce their formation principles, classification, and physical and chemical properties in detail. Then, we summarize their functions in pharmaceutical preparations, such as improving the solubility of insoluble drugs, enhancing the stability of drugs, and promoting the permeability of drugs, as well as their role as active pharmaceutical ingredients (API) to fabricate new drug delivery systems of API-ILs. Finally, we reviewed the applications of ILs in different administration routes, including oral, transdermal, mucosal, and injection routes, and meanwhile offer perspectives for the further use of ILs.