The oral route is the most popular route for the clinical administration of drugs to treat various diseases. Before a drug is absorbed into the blood circulation, it must undergo dissolution and permeation. However, most drugs exhibit poor aqueous solubility, and their limited absorption leads to low oral bioavailability. The solubility of hydrophobic drugs can be improved by various ways, such as solid dispersion, salt formation, pH modification, and self-emulsifying drug delivery system (SEDDS) use. Among them, the SEDDS has garnered attention during recent years as it improves oral bioavailability, reduces drug dose, and increases drug protection from unsuitable environment in the gastrointestinal tract.

SEDDS comprises lipid-based formulations. It can solve the problems related to the dissolution and bioavailability of the Biopharmaceutics Classification System Class II and IV drugs. Depending on the preparation procedure, drug-loaded SEDDS can be divided into micro- (SMEDDS) and nano- (SNEDDS) formulations. In this review, we summarize the classification system of lipid formulations, the mechanism underlying improved oral drug absorption by SEDDS, and recent advances in the SEDDS.

The SEDDS is a potential formulation for drug delivery. Owing to its small particle size, large surface area, high encapsulation efficiency, and high drug loading, the SEDDS can improve the rate and extent of oral absorption by maximizing drug solubility in the intestinal absorption site. Moreover, because of the lipid-based formulation of SEDDS, it can stimulate and enhance lymphatic transport of drugs to avoid hepatic first-pass metabolism, and thus improve their bioavailability.