Hives may be discrete individual lesions or they may join to cover large areas of the body surface. Urticaria may occur because of diverse exposures. Allergens either ingested, injected, or in contact with the skin can cause urticaria, while inhaled allergens are uncommonly involved. Drugs are among the most, frequent identified causative factors. Frequently implicated drugs include penicillin and its derivatives, sulfonamides, aspirin and related anti- inflammatory drugs, and drugs that directly release histamine from mast cells such as codeine and morphine.
Food allergens commonly known to cause hives include milk products, eggs.
Additives for coloring, flavoring, preserving, and adding bulk to foods may cause hives. Physical urticaria results from some physical change in the environment or following contact with some physical factor. Other forms of physical urticaria are seen following stimuli such as cold, local heat, pressure, sunlight, vibration, and stroking or scratching the skin. Dermatographism occurs when hives develop following stroking or mild pressure on the skin. Typically these hives may have a linear pattern and are noted in pressure areas beneath a patient's belt, watchband, or bra strap.