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Less serious side effects of undecylenic acid may include unusual blistering, peeling, itching, redness, dryness, or irritation of treated skin.

It is very unlikely you should have a serious reaction - get emergency medical help if you have any of these signs of an allergic reaction: hives; difficulty breathing; swelling of your face, lips, tongue, or throat. Stop using this medication and call your doctor at once if you have severe burning or stinging of treated skin, or if you have any new skin symptoms.

For more information see http://www.drugs.com/mtm/undecylenic-acid-topical.html