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Urticaria or hives is a relatively common form of allergic reaction that causes raised red skin welts that range upwards from 5 mm (about a 1/4 inch) in diameter, itch severely, and often have a pale border. Urticaria is caused by direct contact with an allergenic substance (generally a plant, such as poison ivy), or an immune response to food or some other allergen. Hives can also be caused by stress. Urticaria is also known as nettle rash or uredo.

Urticaria also includes so called physical allergies. Some physical allergies include those due to water, heat, and the cold. Cold urticaria is the most common physical allergy whose primary symptom is localized hives on an area of the body exposed to the cold. Hives generally worsen when the cold area is warmed. Symptoms usually show up a few minutes after contact with the cold and can last a short while to a few hours.

Drug-induced urticaria has been known to result in severe cardiorespiratory failure. The anti-diabetic suphonylurea glimepiride (trade name AmarylŪ), in particular, has been documented to induce allergic reactions manifesting as urticaria.

Oral antihistamines are one therapy for treating urticaria. They will usually reduce the itching and may help the welts disappear. Oatmeal baths and anti-pruritic lotions can also be used to relieve the itching.

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