[Topical therapy--risks and unwanted effects].

The kinetics of substances topically applied are different from those parenterally or orally applied. Chronic damage difficult to be recognized may be caused by long-term therapy, the high "reservoir effect" of an intact horny layer or the lack of barrier in a damaged horny layer, low substance concentration peaks, or lang diffusion periods in the circulation with very low plasma concentrations. Acute systemic damage is rare with percutaneously applied substances such as salicylic acid; it usually occurs under special conditions. Chronic damage to the skin is more often observed and usually judged by a dermatologist. These changes may not only be caused by active substances but also by excipients including apparently inert supplementary therapies such as the use of surfactants. Because of percutaneous absorption, the changes are either of systemic nature with no relationship to the skin or primarily affecting the skin.