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The efficiency, tolerability, and applicability of topical agents are directly related to employed vehicles. Thus to achieve optimum topical therapy, a solid knowledge of the vehicles, their composition, and their physical and dermato-pharmacological actions are important. Common vehicles are complex mixtures consisting of diverse ingredients belonging to six major groups, i. e. hydrophilic and lipophilic bases, emulsifiers, gel-forming agents, preservatives, and antioxidants. This makes it possible to optimize both the cosmetic features and to adjust a vehicle to the properties of an incorporated drug and site of application. On the other hand it makes it difficult to make a proper choice between several alternatives or to use it in individual prescriptions. In order to simplify the selection of a formulation, it is useful to categorize them systemically into several groups, such as ointments, creams, gels, emulsions, and suspensions. Within these groups some general rules can be derived for the selection of a vehicle with respect to skin conditions and the application site. When active substances are incorporated into a base the dermato-biopharmaceutical properties of the whole system (drug + vehicle) also have to be considered. If for a given vehicle drug transport into the skin does not suffice, several methods are described to facilitate its penetration, such as by hydrating the skin or by adding chemical penetration enhancers.

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