Chronic hives are an inflammation of the skin triggered when certain cells (mast cells) release histamine and other chemicals into your bloodstream, causing small blood vessels to leak. The exact cause of chronic hives isn't well understood — and triggers can be difficult to pinpoint. Chronic hives are thought to be caused by an immune system (autoimmune) disorder and may be linked to another health problem, such as thyroid disease or lupus.

Rarely, a reaction to medication, food, food additives, insects, parasites or infection is identified as an underlying cause of chronic hives. But in most cases, the cause of chronic hives is never identified, even after testing and monitoring symptoms. Heat, cold, pressure, sunlight or other environmental stimuli may worsen chronic hives. Certain pain medications, such as aspirin, ibuprofen (Advil, Motrin, others) and naproxen sodium (Aleve, Anaprox, others), also can worsen chronic hives.

Sep. 17, 2011