How often does aspirin cause or exacerbate urticaria?

Aspirin is rarely a cause of urticaria in an otherwise asymptomatic patient, but many patients with chronic urticaria will have increased whealing if they take aspirin or nonsteroidal antiinflammatory drugs when their disease is active. These same patients are usually able to take aspirin with a much lower risk when their urticaria is inactive, indicating that aspirin is not the cause but a nonspecific exacerbating factor, presumably acting on a pharmacologic basis. Prospective and retrospective data suggest that aspirin administration will cause a flare in 20% to 40% of patients with active urticaria.

Moore-Robinson M, Warin RP: Effect of salicylates in urticaria, BMJ 4:262–264, 1967.