Undecylenic acid USP (5%)

What is undecylenic acid?

Undecylenic acid is an 11-carbon unsaturated fatty acid produced from castor oil. It has been used for decades in over-the-counter antifungal powders, ointments and liquids for athlete’s foot and other skin fungal infections, often combined with its zinc or calcium salts. It is a topical ingredient, not something taken by mouth in consumer products.

FDA’s over-the-counter rules for topical antifungal drug products list undecylenic acid, calcium undecylenate, copper undecylenate and zinc undecylenate as allowed active ingredients, used alone or in any ratio that provides a total undecylenate concentration of 10 to 25 percent (21 CFR 333.210).

What the research says

Most clinical evidence comes from older trials in athlete’s foot (tinea pedis). In a controlled trial of 104 people with laboratory-confirmed athlete’s foot, undecylenic acid powders produced clinical and laboratory cure in 53% of users, compared with 7% of those using talc or no treatment (Smith et al., 1977). A double-blind study of 103 patients found an undecylenic acid ointment and tolnaftate cream performed about the same, and both beat a placebo ointment (Fuerst et al., 1980).

Laboratory research helps explain how it works. In one study, undecylenic acid blocked biofilm formation by Candida albicans yeast and stopped the yeast from switching into its invasive thread-like form, while lowering production of enzymes the fungus uses to damage tissue (Shi et al., 2016).

Toenail fungus is a different problem. The nail plate is thick and hard for topical products to penetrate, and even prescription topical solutions require close to a year of daily use, with modest complete cure rates (Foley et al., 2020). We found no controlled trials of undecylenic acid alone for toenail fungus, so its role there remains unproven.

How it’s used in products

Undecylenic acid appears in drugstore athlete’s foot products and in nail oils marketed online. When a product is sold as an over-the-counter antifungal drug, FDA rules govern its concentration and labeling. Some cosmetic-style nail oils include it at lower levels; for example, one nail oil lists 5%, below the 10 to 25% range set for antifungal drug products.

It is generally well tolerated on intact skin, but it can cause local irritation, stinging or redness, especially on broken skin. Keep it away from the eyes, and do not swallow it. Anyone with diabetes, poor circulation, a weakened immune system, or a foot infection that is painful, red, swollen or spreading should see a clinician rather than self-treat. For nail changes, a dermatologist or podiatrist can confirm whether fungus is actually the cause, since other nail conditions can look similar (AAD).