The term derives from the Latin trans (across) and the Greek derma (skin). In pharmacology and pharmacokinetics, it describes a route of administration in which a drug or other compound diffuses across the stratum corneum , the outermost layer of the epidermis , and subsequently enters the dermis, subcutaneous tissue, and capillary network, from which it distributes systemically. This distinguishes the transdermal route from purely topical preparations, where absorption into the bloodstream is not the primary objective.
Skin permeability is governed by several physicochemical factors, including the molecular weight of the compound, its lipophilicity (oil-solubility), ionisation state, and concentration gradient across the skin barrier. Molecules with a molecular weight below approximately 500 daltons and moderate lipophilicity tend to cross the stratum corneum most readily. Formulation strategies , including the use of chemical penetration enhancers, carrier systems such as liposomes, or physical methods such as microneedle arrays; are employed in research and pharmaceutical development to improve transdermal flux for compounds that do not cross the barrier efficiently on their own.
In the Australian regulatory context, transdermal medicines are assessed by the Therapeutic Goods Administration (TGA) under the Therapeutic Goods Act 1989. Scheduling under the Poisons Standard (the current version of which is published on the Federal Register of Legislation) determines whether a transdermal formulation is available over the counter, as a pharmacist-only medicine, or only on prescription. Some transdermal preparations containing controlled substances are listed as Schedule 8 (controlled drugs) under the Poisons Standard, with supply and prescription requirements varying by jurisdiction.
The transdermal route is studied across a range of compound classes, including pharmaceuticals, cannabinoids, and peptides. Research interest lies partly in its potential to bypass first-pass hepatic metabolism, the process by which orally administered compounds are partially broken down in the liver before reaching systemic circulation, and to provide more sustained plasma concentration profiles compared with oral dosing. The clinical and regulatory status of any specific transdermal product depends entirely on its scheduling classification and TGA approval status; unapproved transdermal products supplied in Australia may be subject to enforcement action.
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