Undecylenic Acid for Nails: What the FDA Context Actually Says
Undecylenic acid is a real OTC antifungal active ingredient — but the approved/monograph context matters. Its recognized OTC uses for superficial skin fungal infections should not be stretched into an unsupported nail-fungus claim.
What FDA Monograph M005 covers
The FDA monograph allows undecylenic acid/undecylenates within a specified concentration range for certain topical antifungal skin products. The same monograph's directions state that products labeled under those conditions are not effective on the scalp or nails.
Why this distinction matters
Consumers often see “antifungal active” and assume “proven for toenail fungus.” Nails are a much harder drug-delivery target than skin. The regulatory language is a reminder to check the exact indication rather than transferring a skin claim to a nail.
Kerassentials context
Undecylenic acid is one of the ingredients highlighted by the Kerassentials merchant page. We do not know the concentration from the public text presentation we reviewed, so we do not infer that it matches an OTC monograph concentration or that the final formula is an approved drug.
Primary source
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Last reviewed: September 28, 2026