Undecylenic Acid
In plain terms
A fatty acid derived from castor oil, and present in human sweat, used mainly as an over-the-counter antifungal for athlete's foot and similar infections. It is well tolerated, with mild passing irritation the main thing some people notice at treatment strength.
What the evidence shows
Undecylenic acid is an eleven-carbon fatty acid made from castor oil, and the body already makes a little of it in sweat. Its main use is as an antifungal: the FDA lists it as a safe and effective over-the-counter treatment for athlete's foot and related infections at 10 to 25%, and it also appears at lower levels in cosmetics. On safety it reads as a plain fatty acid. The US cosmetic-ingredient panel reviewed the fatty acids and found undecylenic acid is not a skin sensitizer, with laboratory and animal tests pointing the same way, and its structure is not one that reacts with skin proteins. It carries no hormone, cancer, or environmental concern and is cleared as a food flavoring. The one thing people do report, at the higher antifungal strengths, is mild local irritation, redness, itching, or a little burning, which settles once the product is stopped; true allergy to it is documented but rare. For everyday exposure this sits in the no-real-concern column, with the irritation a use note rather than a safety problem.
On the label
Listed as undecylenic acid or its zinc and copper salts, named on the front of athlete's-foot products and lower in cosmetic ingredient lists. At antifungal strength it can sting or redden skin for a short while; stop if that does not settle, and it is fine to use on intact skin rather than broken or raw areas.