Many medications can cause hives, but only about 10% of hives are caused by medications. Hives will most often occur in the first 36 hours after starting the medication, but hives can occur even after taking a medicine for a long time. You can see that antibiotics are a common culprit.
Hives caused by radiocontrast dye, codeine, morphine, and aspirin are usually not triggered by the immune system, rather these drugs cause the direct release of histamine from specialized white blood cells called mast cells.
