Hives (urticaria) are itchy, red, swollen patches of skin that appear suddenly. Although they may vanish within one to two hours, they can last as long as two days. They frequently occur in clusters, with new clusters forming as others fade. They do not leave scars.

Hives may have an allergic or non-allergic cause; and the exact culprit usually cannot be determined. They are often triggered by a food or a drug. Common foods that cause hives include nuts, shellfish, peanuts, and eggs. Drugs that typically produce hives include penicillin, sulfa, anticonvulsants, aspirin, and nonsteroidal anti-inflammatory drugs (NSAIDs). Tight-fitting clothing that scratches or rubs the skin, activities that increase the body’s temperature (warm baths, hot tubs, exercise, or fever), and exposure to cold or sunlight may all cause hives in susceptible individuals.

Angioedema is a condition like hives that occurs in the deeper layers of the skin, most often in the hands, feet, and face. Certain types of hereditary angioedema may cause fatal swelling of the larynx.

Hives and angioedema are treated by avoiding whatever substance has provoked the outbreak, if it can be identified.