Arguably one of the greatest medical advancements in the 20th Century was the discovery of insulin. Physiologically, insulin
is a potent hypoglycaemic agent, which is vital in order for plasma glucose levels to be maintained within the normal range of 4-
7mmol/L. Various attempts have been made in the production, purification, formulation and methods of delivery of insulin. Despite alternative
routes being investigated, these routes have met with limited success. Although the pulmonary route offers potential for the delivery
of polypeptide drugs due to its large surface area for insulin absorption, it has low bioavailability. Microencapsulation and nanoencapsulation
exhibit potential progress in insulin delivery although delivery is at an early stage. In this Review, we discuss the structure of
insulin and the physiological importance in addition to the injectable and non-injectable methods used. The currently available insulin
therapies are presented and the clinical importance of such therapies.