Symptoms of ordinary hives
Ordinary hives flare up suddenly and usually for no specific reason. Welts appear, often in several places. They flare, itch, swell, and go away in a matter of minutes to hours, only to appear elsewhere. This sequence may go on from days to weeks. Most episodes of hives last less than six weeks. Although that cutoff point is arbitrary, hives that last more than six weeks are often called "chronic."
Causes of ordinary hives
As noted above, many cases of ordinary hives are "idiopathic," meaning no cause is known. Others may be triggered by viral infections. A few may be caused by medications, usually when they have been taken for the first time a few weeks before. (It is uncommon for drugs taken continuously for long periods to cause hives or other reactions.)
Some medications, like morphine, codeine, aspirin, and other nonsteroidal anti-inflammatory drugs (NSAIDs, such as ibuprofen), cause the body to release histamine and produce urticaria through nonallergic mechanisms.
Despite the reputation hives have for being "allergic," when there is no obvious connection between something new that a person has been exposed to and the onset of hives, allergy testing is not usually helpful.
Chronic hives can last from months to years. Allergy testing and laboratory tests are hardly ever useful in such cases.