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Heparin is a carbohydrate anticoagulant used to treat blood clots, and is associated with several side effects including bleeding from various organs, most commonly the skin. There have been reports of two types of adverse effects of heparin on the skin. Heparin may cause necrosis, or tissue death, due to abnormal changes in blood clotting, and also the formation of erythematous, infiltrated plaques, which are red, inflamed, itchy patches of skin. These abnormal skin patches resemble contact dermatitis, or skin inflammation resulting from exposure to an irritating substance. The plaques are usually formed at the sites where heparin is injected and most commonly occur in women. The mechanism underlying the formation of the heparin plaques is not known, but may involve a form of tissue deterioration, a metabolic response, or a delayed hypersensitivity reaction (a physical response resulting from an abnormal sensitivity to heparin). Fifteen patients, 14 women and one man, developed erythematous, infiltrated plaques within three to 21 days after heparin was injected beneath the skin. The results of various skin tests, clinical findings, and examinations of skin tissue structure indicated that these abnormal skin lesions were the result of a hypersensitivity reaction to heparin. The skin plaques resulted from heparin in all the female patients, but were caused by chlorocresol in the only male patient. (Consumer Summary produced by Reliance Medical Information, Inc.)