Many patients experience adverse reactions to medications. Only a small percent of these reactions, however, are true allergic reactions. An allergic reaction means the patient’s immune system is programmed to recognize a certain medication and produce a specific reaction whenever it encounters that drug.

The most potentially severe allergic reaction to a drug is anaphylaxis. This happens when the patient, unknowingly, has a large amount of an allergy protein (called IgE antibody) in his or her body specific for a drug, such as penicillin.