A transdermal drug delivery system (TDDS) is a method of delivering drugs systemically by applying a drug formulation onto intact and healthy skin. It is a non-invasive method of drug delivery that represents an attractive alternative to oral delivery of drugs and is poised to provide an alternative to hypodermic injection. TDDS is designed to deliver a therapeutically effective amount of a drug across the skin for systemic distribution. The skin is a large and logical target for drug delivery, but its basic functions limit its utility for this purpose. The skin functions mainly to protect the body from external penetration and to contain all body fluids. There are two important layers to the human skin: the epidermis and the dermis. For transdermal delivery, drugs must pass through the two sublayers of the epidermis to reach the microcirculation of the dermis. There are two main pathways by which drugs can cross the skin and reach the systemic circulation: the more direct route is known as the transcellular pathway, and the other is the paracellular pathway.

TDDS should be formulated to provide the maximum thermodynamic driving force for passive diffusion across the skin, which is saturated with a sufficient payload of the drug to ensure delivery of drugs across the skin. The ability of approved transdermal drugs to penetrate the skin varies widely from the extremely permeable nicotine to compounds, such as buprenorphine and the progestins, which have very low predicted fluxes. The first transdermal patch approved for systemic delivery in 1979 was a patch for the sustained, three-day delivery of scopolamine in the treatment of motion sickness. Transdermal delivery is currently restricted to approximately 17 drug molecules that are approved by the US Food and Drug Administration (FDA) .

Recent studies have focused on aspects of transdermal drug delivery technologies ranging from the development of chemical enhancers that increase the spread of drugs across the skin or increase the solubility of drugs in the skin to novel innovative approaches that extend this concept to the design of super-strong formulations, microemulsions, and vesicles. TDDS supplemented by appropriate equipment is termed as active transdermal delivery, which is known to deliver drugs quickly and reliably into the systemic circulation.