Causes

The welts that come with hives arise when certain cells release histamine and other chemicals into your bloodstream.

Doctors often can't identify the reason for chronic hives or why acute hives sometimes turn into a long-term problem. The skin reaction may be triggered by:

• Pain medications
• Insects or parasites
• Infection
• Scratching
• Heat or cold
• Stress
• Sunlight
• Exercise
• Alcohol or food
• Pressure on the skin, as from a tight waistband

In some cases, chronic hives may be related to an underlying illness, such as a thyroid disease or, rarely, cancer.