The Effect of Sequential Topical Application of Dermatologic Medications on Absorption: Clinical Considerations

Sequential application of topical dermatologic medications - defined as applying two or more agents to the same anatomical site within a short interval - is common in dermatologic therapy. Despite widespread clinical use, the pharmacologic consequences of application order, timing, and vehicle interactions on percutaneous absorption remain incompletely characterized.

This review synthesizes clinical, in vivo human volunteer, and mechanistic in vitro and ex vivo evidence evaluating the impact of sequential topical application on cutaneous drug absorption and bioavailability. A qualitative literature search identified twelve eligible studies, including randomized controlled trials, in vivo human studies, and experimental diffusion and visualization models. Across heterogeneous study designs, sequential application was not pharmacologically neutral. Alterations in percutaneous absorption were driven by changes in stratum corneum barrier properties and vehicle or excipient interactions rather than formulation class alone. Sequencing effects were most pronounced when the first-applied product induced keratolysis, hydration or occlusion, or lipid film formation, or when vehicle redissolution and redistribution altered the effective treated surface area. In barrier-compromised conditions, such as atopic dermatitis, changes in application order were less likely to translate into measurable clinical effects. Fixed-combination formulations generally demonstrated more consistent and predictable cutaneous delivery than free sequential regimens.

Sequential topical application can significantly influence percutaneous absorption through barrier modification and vehicle interactions. Pharmacokinetic effects are context-dependent and cannot be inferred solely from formulation type. Mechanism-informed sequencing and judicious selection of compatible vehicles may improve predictability of topical drug delivery, highlighting the need for standardized, pharmacologically focused studies to guide application order and timing.