Undecylenic acid (or undecenoic acid) is a medium chain saturated fatty acid similar to caprylic acid. The main source is castor bean oil. It has strong antifungal properties and specialist nutritional supplements containing undecylenic acid are often prescribed by nutritionists and naturally orientated doctors for the treatment of Candida related illnesses such as intestinal overgrowth and thrush. Undecylenic acid was the antifungal of choice in general medical practice before modern drugs were developed and remains popular as the active ingredient in topical antifungal preperations for the treatment of fungal skin infections such as athlete's foot and jock itch.

Undecylenic acid is reported to be up to 6 times as potent as caprylic acid. Thorne Research who manufacture the undercylenic acid nutritional supplement Formula SF 722 state that its molecular structure makes undecylenic acid the most potent antifungal of all the fatty acids.

Undecylenic acid works against fungal infections such as those caused by Candida Albicans via a number of mechanisms. Research has demonstrated that it kills Candida and other fungi directly as well as inhibiting the conversion of the yeast form of these organisms to the more invasive hypael form. It is this conversion that environmental medicine specialists believe contributes to the development of leaky gut syndrome and the spread of Candida to other parts of the body from the intestines.

Undecylenic acid supplements are not as common as their caprylic acid counterparts but prices do not differ considerably.