This guidance provides recommendations for the conduct of in vivo absorption trials for topically applied active ingredients that are under consideration for inclusion in an over-the-counter (OTC) drug monograph. A Maximal Usage Trial (MUsT) is a standard approach to assess the in vivo bioavailability of topical drug products intended for local therapeutic effects. , The methodology described in this guidance adapts MUsT principles for active ingredients being considered for inclusion in an over-the-counter (OTC) monograph. Because information from a MUsT can help identify the potential for systemic exposure to a topically applied active ingredient, such information can help inform an FDA determination of whether additional safety data are needed to support a finding that a topical OTC drug containing that active ingredient is generally recognized as safe and effective (GRASE) for its intended use.

This guidance outlines the FDA’s recommendations for designing and conducting a MUsT for this purpose, including critical study elements, data analysis, and considerations for special topic areas (e.g., pediatrics, geriatrics). This guidance also encourages study sponsors to seek feedback from the FDA on their overall approach and the design of a particular study.