In general, adverse reactions to drugs are not uncommon, and almost any drug can cause an adverse reaction. Reactions range from irritating or mild side effects such as nausea and vomiting, to allergic reactions, including life-threatening anaphylaxis. Some drug reactions are idiosyncratic (unusual effects of the medication). For example, aspirin can cause nonallergic hives (no antibodies formed), or it may trigger asthma. Only a small proportion of these reactions are allergic in nature. Many individuals may confuse an uncomfortable but not serious side effect of a medicine, such as nausea, with a true drug allergy, which can be life-threatening. True drug allergies occur when there is an allergic reaction to a medication. This is caused by hypersensitivity of the immune system, leading to an incorrect response against a substance that is harmless in most people. The body becomes sensitized (the immune system is triggered) by the first exposure to the medication. The second or subsequent exposure causes an immune response, including the production of antibodies and release of histamine. Most drug allergies cause minor skin rashes and hives. However, other symptoms occasionally develop and life-threatening acute allergic reaction involving the whole body (anaphylaxis) can occur. Serum sickness is a delayed type of drug allergy that occurs a week or more after exposure to a medication or vaccine. Penicillin and related antibiotics are the most common cause of drug allergies. Other common allergy-causing drugs include sulfa drugs, anticonvulsants, insulin preparations (particularly animal sources of insulin), local anesthetics such as Novocain, and iodine (found in many x-ray contrast dyes).
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