Although many people claim to be "allergic" to aspirin, most describe their "allergy" as abdominal pain or heartburn. These common side effects are not allergies, but rather reflect the irritating effects of aspirin on the lining of the stomach. True allergy to aspirin occurs in fewer than 1 out every 100 persons. True allergy may include hives, itching, swelling, or difficulty breathing. Allergy to aspirin is more common in persons with asthma, allergic rhinitis, or nasal polyps. In such persons, the risk of an allergic reaction to aspirin may be as high as 1 to 3 persons out of every 10. People with true aspirin allergy also may be allergic to other compounds. One such compound is tartrazine, or yellow dye #5. This dye is used in many medicines, foods (such as soft drinks), and candy or desserts. In addition, people with an allergy to aspirin often are allergic to other NSAIDs, such as ibuprofen.

http://www.medicinenet.com/analgesics_antipyretics/page2.htm