Hives occur when something prompts cells to release histamine, a chemical found in the skin. The histamine causes nearby blood vessels to dilate. Fluid leaks out of the dilated vessels and collects under the skin in a raised, flushed, itchy bump called a wheal or hive. Some wheals look like mosquito bites. Wheals often come in groups and may be as small as pencil erasers or as large as a dinner plate.
Some people know that certain foods or drugs give them hives. For most others, the causes may not be obvious.
Acute hives, or hives that are a reaction to a stimulus, such as a drug, can last for hours or days. Chronic hives, often of unknown cause, can last for weeks or months.
Some foods that occasionally cause hives are peanuts and other nuts, eggs, beans, chocolate, strawberries and other berries, spices, freshwater fish, shellfish, milk, wheat, cheese, and tomatoes.
Drugs that can cause hives include penicillin, sulfa drugs and codeine.
Extensive hive breakouts can be very serious, especially when hives form on the lips and in the throat, interfering with breathing and swallowing. Shock—in which severe swelling, dizziness and even loss of consciousness occur—can accompany widespread hives.
Self-care for hives
Take an oral antihistamine; be aware of drowsiness.
Try a topical over-the-counter Hydrocortisone cream.
Soak in a lukewarm or cool bath with one cup of baking soda or an oatmeal product.
If hives develop after a bee sting or an insect bite, see a health care provider. You may need a prescription kit to counteract the reaction. A kit contains an injectable dose of epinephrine.