Transdermal drug delivery (TDD) has become an alternative to conventional routes, like oral or nasal administration, because of its avoidance of their side effects. The principal barrier to TDD, the stratum corneum (SC) in human skin, makes penetration enhancers (PEs) like essential oils (EOs) necessary for drugs to cross the outermost layer. EOs can penetrate through the skin and thus play a major role in enhancing drug permeability through human skin because of their interaction with the

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... bilayers. This review deals with different essential oils, their constituents - especially monoterpenes and sesquiterpenes with long chain alkyl functionality – how and why they enhance the penetration of special drugs, their mechanism of action and their advantages in contrast to oral administration.