Measures to increase transdermal absorption have always been considered a "double-edged sword". These measures may enhance the desired pharmacological outcome. For example, a highly potent topical corticosteroid in an ointment base, following skin hydration and body occlusion, will do wonders for widespread psoriasis. On the contrary, all of these measures significantly increase the systemic absorption of topical corticosteroids, possibly producing a net prednisone-like effect of topical corticosteroids. In short order, the trade-offs are relatively few. After 2-3 weeks or more, this poor method of chronic topical corticosteroid administration may produce important systemic adverse effects such as weight gain, fluid retention, hypertension, hypokalemia, and Cushing-like Variety. It is important to note that the absorption of all topical drugs occurs by passive diffusion. diazolidinyl urea
Immediate-type hypersensitivity (Coombs-Gell Type I) reactions are associated with topical applications in a variety of clinical settings. In particular, topical application to ulcerated skin allows the almost immediate entry of the applied drug into the systemic circulation. In this setting, allergic reactions to topical bacitracin or neomycin have been reported. Likewise, mucosal application of drugs (such as eye drops, vaginal suppositories, and rectal foams or suppositories) can result in significant systemic levels of various drugs and freedom from "first-pass effects" due to the small intestine and liver. Although the risks of topical administration at these sites are generally minimal, clinicians should always be aware of this possibility of systemic absorption.
Much of the art and science of dermatology revolves around selecting the appropriate carrier for topical application (Tables 1-11). In general, the choice of carrier is as important as the selection of an appropriate active ingredient. Two common consequences for certain vehicles are as follows:
1
Irritation: Most notable from high concentrations of propylene glycol; other "alcohols" or certain acidic carrier ingredients may also be irritating, especially when applied to diseased skin with altered barrier function.
2
Contact allergy/sensitization: preservatives commonly found in various water-based (creams, lotions, solutions) topical products, including various parabens and "formalin-releasing agents" (eg, quaternium-15 , imidazolidinyl urea and diazolidinyl urea).