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Undecylenic Acid: What It Treats, Why It Is Not Approved for Nails, and Safety
Undecylenic acid is a fatty acid made from castor oil and an approved over-the-counter antifungal for athlete’s foot, jock itch and ringworm at strengths of 10% to 25%. It is not approved for nail fungus.
Last updated 2 November 2026
Undecylenic Acid: the short answer
Undecylenic acid is a fatty acid made from castor oil. In the US it is an approved over-the-counter antifungal for athlete’s foot, jock itch and ringworm, at a total strength of 10% to 25%, applied twice a day for 2 to 4 weeks. It is not approved for nail fungus: approved products must say on the label that they are not effective on the scalp or nails. It can irritate the skin or cause allergic reactions. Capsules sold for candida have only laboratory studies and case reports behind them.
| What it is | An 11-carbon fatty acid made from castor oil, used with its zinc and calcium salts in antifungal skin products. |
|---|---|
| Approved for | Athlete’s foot, jock itch and ringworm, at a total of 10% to 25% undecylenate, twice a day for 2 to 4 weeks (US over-the-counter rules). |
| Not approved for | Nail or scalp fungus: approved products must say they are not effective on the scalp or nails. |
| Capsules for candida | Laboratory studies and case reports only; no controlled trials found. |
| Safety | Can cause stinging, irritation or allergic reactions on the skin. Not for children under 2 or nappy rash without a doctor’s advice. |
| In products we review | Kerassentials, KeraFirm Pro |
Sources: 21 CFR 333.210: Antifungal active ingredients (US over-the-counter rules) · 21 CFR 333.250: Labeling of antifungal drug products
Benefits of Undecylenic Acid
What is undecylenic acid?
Undecylenic acid (10-undecenoic acid) is an unsaturated fatty acid with 11 carbon atoms. It is made by heating ricinoleic acid, the main fatty acid of castor oil. It and its zinc and calcium salts (undecylenates) have been used for decades in antifungal creams, powders, sprays and solutions for the skin.
Why people use it
It is used on the skin to treat athlete’s foot, jock itch and ringworm. It is also an ingredient in some nail oils and solutions sold for fungal nails, and in capsules sold for “candida” yeast overgrowth.
Top Rated Supplements with Undecylenic Acid
Products we review that list Undecylenic Acid as an ingredient. Not every product discloses how much of each ingredient a serving contains, so check each review before assuming it matches the amounts used in studies.
Kerassentials is a nail oil sold online for toenail fungus, to be brushed on four times a day. This review covers its full 17-ingredient label, what research shows for its main ingredients, how it compares with treatments that are proven to work, the sales claims, the prices and the 60-day refund.
KeraFirm Pro is a nail liquid sold online for toenail fungus, brushed on four times a day. This review compares its 9-ingredient label with the 13 ingredients on its sales page, covers what research shows for the main ones, which treatments are proven to work, the sales claims, the prices and the 60-day refund.
Scientific Analysis of Undecylenic Acid
What it is approved for
In the US, the FDA’s rules for over-the-counter antifungal products list undecylenic acid and its calcium, copper and zinc salts as approved active ingredients when they provide a total of 10% to 25% undecylenate. Such products are approved for athlete’s foot, jock itch and ringworm. The approved directions are to clean and dry the affected area, then apply a thin layer twice a day, morning and night: for 4 weeks for athlete’s foot and ringworm and for 2 weeks for jock itch, and to see a doctor if the problem persists. The label also warns not to use the product on children under 2 or for nappy (diaper) rash.
Does it work for nail fungus?
It is not approved for nail fungus. The FDA rules require approved products to say on the label: “This product is not effective on the scalp or nails.” Fungus under a toenail is hard to reach through the hard nail plate, and we found no good trials of undecylenic acid for it. Thick or discoloured nails are not always fungal, so a doctor or podiatrist should confirm it first; if it is fungus, prescription tablets such as terbinafine and nail solutions such as efinaconazole have the best evidence. A toenail takes a year or more to grow out, so any treatment is slow.
Capsules for candida
Undecylenic acid capsules are sold online for “candida overgrowth”. In laboratory studies it stops Candida yeast from forming the thread-like shapes and biofilms it uses to invade tissue, but the evidence for taking it by mouth comes from case reports, and we found no controlled trials. Yeast infections should be diagnosed and treated by a doctor, and the symptoms often blamed on “candida overgrowth” have many other causes.
Side effects and safety
On the skin it can cause stinging, irritation and allergic reactions; allergic skin reactions have been reported with nail solutions containing it. Stop using it if your skin becomes red, itchy or sore, and keep it away from the eyes. Taken by mouth in large amounts it can upset the stomach. If you have diabetes or poor circulation, see a doctor about foot problems instead of treating them yourself.
Products we review that contain it
Kerassentials (first of 17 ingredients) and KeraFirm Pro (second of 9) both contain 5% undecylenic acid, half the lowest strength approved even for athlete’s foot, and both are sold for toenail fungus, which approved undecylenic acid products are not. Both also contain tea tree oil.
Frequently Asked Questions
A fatty acid made from castor oil. With its zinc and calcium salts, it is an approved over-the-counter antifungal for athlete’s foot, jock itch and ringworm.
Mainly for athlete’s foot, jock itch and ringworm, in creams, powders and solutions at 10% to 25%. It also appears in nail oils sold for fungal nails and in capsules sold for candida, neither of which is an approved use.
It is not approved for it: US rules require approved products to say they are not effective on the scalp or nails, and we found no good trials for nail fungus. Prescription tablets and nail solutions have the best evidence.
Approved products contain a total of 10% to 25% undecylenic acid and its salts, applied twice a day for 4 weeks for athlete’s foot and ringworm or 2 weeks for jock itch. Products with 5% are below the approved range.
On the skin it is generally safe, but it can cause stinging, irritation and allergic reactions. Stop if the skin becomes red or sore, keep it away from the eyes, and do not use it on children under 2 without a doctor’s advice.
Capsules are sold for this, but the evidence is limited to laboratory studies and case reports, with no controlled trials. Yeast infections should be diagnosed and treated by a doctor.
Among the products we review, the nail oils Kerassentials and KeraFirm Pro both contain 5% undecylenic acid, half the lowest strength approved for athlete’s foot.
⚖️ Conclusion
Undecylenic acid is a genuine antifungal with a long record on the skin: at 10% to 25%, applied twice a day for a few weeks, it is an approved treatment for athlete’s foot, jock itch and ringworm. It is not approved for nail fungus, and approved products must say so on the label. A nail oil with 5%, half the lowest approved strength, is unlikely to reach fungus under a thick toenail. For athlete’s foot, an approved 10% to 25% product is a reasonable choice; for fungal nails, see a doctor or podiatrist.