• In acute urticaria, the literature suggests that a cause may be identified in around 50% of cases. In practice, however, the cause is rarely obvious at the first presentation. Consider:
• Allergy to:
• Food such as nuts, strawberries, eggs, additives, spices, shellfish.
• Insect bites and stings — for more information, see the CKS topic on Insect bites and stings.
• Drugs such as penicillins, aspirin, nonsteroidal anti-inflammatory drugs (NSAIDs), vaccinations, and angiotensin-converting enzyme (ACE) inhibitors. A systematic review showed that skin reactions related to drugs are rare (less than 5%), and that NSAIDs cause 5–14% of cases of drug-induced urticaria [Bigby, 2001; Kozel and Sabroe, 2005].
• A viral infection, such as an upper respiratory tract infection.
• Skin contact with chemicals, latex, cosmetics, plants, ointments, or nettle stings (these cause a localised area of urticaria).
• Physical stimuli:
• The most common physical urticaria is dermatographism. It affects 5% of the population, with a rash that develops on an area of skin that has been rubbed firmly.
• Delayed-pressure urticaria is when tender weals (lasting 24–72 hours) develop between 30 minutes and 12 hours after pressure to the skin.
• Cholinergic urticaria is when pin-point weals (on an erythematous background) develop as a secondary response to increases in the core body temperature related to exercise, warmth, spicy foods, drinks, and emotional stress.
• Other causes of physical urticaria include low temperatures, sunlight, and water.

[Kobza-Black and Champion, 1998; Wanderer et al, 2000; Kozel and Sabroe, 2005]