What factor primarily determines the potency of a topical corticosteroid?

The potency of a topical corticosteroid is primarily determined by the vehicle of delivery. The formulation of the drug can significantly influence its absorption through the skin and, consequently, its effectiveness. Different vehicles such as creams, ointments, lotions, gels, and elixirs have varying characteristics that affect how much of the corticosteroid penetrates the skin barrier.

For instance, ointments are typically more occlusive than creams or lotions, which can result in better drug absorption and enhanced potency, especially in inflamed or thickened skin. In contrast, lotions may be less potent due to their higher water content and faster evaporation, which can reduce the amount of medication available for absorption. Therefore, the choice of vehicle is crucial in determining the level of therapeutic effect achieved with a particular topical corticosteroid.

While duration of use and site of application can impact the effectiveness and safety of topical corticosteroids in a broader sense, they do not primarily dictate the inherent potency of the corticosteroid itself. Instead, they may influence clinical outcomes or the potential for side effects when the corticosteroid is applied.