Causes

Hives and angioedema can be caused by:

• Foods. Many foods can trigger reactions in people with sensitivities. Shellfish, fish, peanuts, tree nuts, soy, wheat, eggs and milk are frequent offenders.
• Medications. Many medications may cause hives or angioedema. Common culprits include penicillin, aspirin, ibuprofen (Advil, Motrin IB, others), naproxen sodium (Aleve) and blood pressure medications.
• Airborne allergens. Pollen and other allergens that you breathe in can trigger hives, sometimes accompanied by upper and lower respiratory tract symptoms.
• Environmental factors. Examples include sunlight, vibration such as from using a lawn mower, hot showers or baths, pressure on the skin such as from tight clothing or scratching, emotional stress, insect bites and exercise.
• Medical treatments or underlying conditions. Hives and angioedema also occasionally occur in response to blood transfusions and infections with bacteria or viruses such as hepatitis and HIV.

Oftentimes, no specific cause can be identified, especially in the case of chronic hives.