Adverse reactions to drugs are common. (adverse means unwanted or unexpected.) Almost any drug can cause an adverse reaction. Reactions range from irritating or mild side effects such as nausea and vomiting to life-threatening anaphylaxis.
A true drug allergy is caused by a series of chemical steps in the body that produce the allergic reaction to a medication.
The first time you take the medicine, you may have no problems. However, your body's immune system may produce a substance (antibody) called IgE against that drug. The next time you take the drug, the IgE tells your white blood cells to make a chemical called histamine, which causes your allergy symptoms. A drug allergy may also occur without your body producing IgE. Instead, it might produce other types of antibodies, or have other reactions that do not produce antibodies.
Most drug allergies cause minor skin rashes and hives. Serum sickness is a delayed type of drug allergy that occurs a week or more after you are exposed to a medication or vaccine.
Common allergy-causing drugs include:
Insulin (especially animal sources of insulin)
Iodinated (containing iodine) x-ray contrast dyes (these can cause allergy-like reactions)
Penicillin and related antibiotics
Most side effects of drugs are not due to an allergic reaction. For example, aspirin can cause nonallergic hives or trigger asthma. Some drug reactions are considered idiosyncratic. This means the reaction is an unusual effect of the medication. It is not due to a known chemical effect of the drug. Many people confuse an uncomfortable, but not serious, side effect of a medicine (such as nausea) with a true drug allergy.