A person who has a drug allergy develops allergic symptoms because the immune system over-reacts to a drug.

The immune system is made up of special cells that fight infection. When the body is exposed to the drug, the cells release chemicals into the blood stream. The chemicals cause inflammation of the skin, nose, throat, lungs or intestines.

Allergy symptoms include itching, hives, rash, diarrhea, sneezing, runny nose, or wheezing.