Topical formulations are a mainstay treatment for skin disorders, as they offer targeted dose of medication where needed and thereby reduce side effects and toxicity to other organs. Absorption through the skin depends on properties of the skin and the medication. Skin thickness and hydration can vary considerably depending on individual age and skin disorder as well as the particular body site of interest. Generally, absorption occurs more readily in thin and well-hydrated skin.

Chemical properties of the drug, including molecular size, lipophilicity, and concentration of active ingredients, also influence medication absorption. Smaller sized particles, greater lipophilicity, and higher concentrations are associated with better penetration and absorption.

Consequently, the practitioner must choose the topical formulation of choice optimized for a particular site of the body or type of skin condition. For example, a medication for chronic scaly skin lesions should have high lipophilicity and moisturizing properties to maximize penetration of active ingredients into the skin. In general, conditions with acute inflammation are treated with aqueous drying preparations, whereas chronic inflammation is treated with greasier, more lubricating compounds.

Topical drugs are chosen based on their active ingredients and vehicle (base which includes inactive ingredients), as either variables can be manipulated to optimize potency of treatment. While the medication itself will directly target the condition, the vehicle has an equally important (though often overlooked) role in altering the composition and overall delivery of the active ingredients. In fact, differing vehicles between generics and their corresponding brand name drug cause decreased potency despite having equivalent active ingredients. This section will provide an overview of the different types of dermatologic topical preparations, including the following formulations (in order of most drying to hydrating): powders, liquids, gels, creams, and ointments. Though most topical medications fall into one of the aforementioned categories, a select number do not; this section will also touch upon these few.

A. Powders are dried forms of substances, with either inert or active ingredients, used primarily in intertriginous areas to reduce moisture, maceration, and friction. They may be nonabsorptive, making the skin slippery (talc), or absorptive (starch). Talc can cause a granulomatous reaction in wounds, and starch may be metabolized by organisms to cause an increase in Candida overgrowth. Powders should be used only in intact skin and avoided at any site with maceration, ulceration, or erosion.

B. Gels are drying agents best used in hairy or oily areas. These thickened semisolid emulsions liquefy upon contact with the skin, usually drying to
a thin and greaseless film. Since gels often contain alcohol and/or acetone, they often cause relatively high rates of irritation. Therefore, they should be avoided in sites with fissures, erosions, or macerations. The skin does not absorb gel as well as other preparations. Gels are often prescribed for conditions involving hands, feet, and the trunk.