10-Undecenoic acid, more commonly known as undecylenic acid, is a fatty acid derived from castor oil. Its use in treating fungal infections—particularly those affecting the skin, such as athlete’s foot (tinea pedis) and ringworm (tinea corporis)—has scientific support. The mechanism of action involves disruption of fungal cell membranes, leading to leakage of cellular contents and inhibition of fungal growth. Several in vitro studies have demonstrated antifungal activity against dermatophytes (e.g., Trichophyton species), which are common causative agents of superficial fungal infections.

Undecylenic acid is approved by the US FDA as an over-the-counter topical treatment for certain fungal skin infections, reflecting both established safety and efficacy for these uses. However, the clinical evidence is largely limited to older studies and post-marketing experience, rather than large, modern randomized controlled trials. Some contemporary reviews acknowledge its efficacy but note that it may be less potent than newer antifungal agents (such as azoles or allylamines). Nonetheless, undecylenic acid remains in use in topical formulations, where it is generally well-tolerated.

In summary, while undecylenic acid is a scientifically validated treatment for superficial fungal infections, the strength of evidence is moderate (rated 3/5), largely based on historical clinical use, regulatory approval, and in vitro studies.