Hives, also known as urticaria, are raised red bumps on the skin caused by the release of histamine and other inflammatory substances. Sensations with these lesions include itch, stinging, prickling, and pain may occur with these lesions. Urticaria can be triggered by many factors, the most common of which is an upper respiratory tract infection. Drugs are the 2nd most common cause, often barbiturates, aspirin, non-steroidal anti-inflammatories (NSAIDs), opiates, alcohol, antibiotics, blood pressure medications and radiographic contrast media are implicated. Foods cause urticaria in 1% of cases. Other causes include systemic illnesses/infections, blood disorders, pregnancy, and physical stimuli (such as heat, cold, water, pressure, vibration). Unfortunately, in the overwhelming majority of cases, the cause of urticaria is not found. This is called idiopathic urticaria. Each individual lesion typically lasts less than 24 hours, but the condition may last much longer.