An allergic reaction is caused by the body’s immune system overreacting to the drug, which is viewed as a chemical “invader,” or antigen. This overreaction is often called a hypersensitivity reaction.

• The body produces antibodies to the antigen and stores the antibodies on special cells.
  • The antibody in an allergic reaction is called immunoglobulin E, or IgE.
  • When the body is exposed to the drug again, the antibodies signal the cells to release chemicals called “mediators.” Histamine is an example of a mediator.
  • The effects of these mediators on organs and other cells cause the symptoms of the reaction.
• The most common triggers of drug allergies are the following:
  • Painkillers (called analgesics) such as codeine, morphine, nonsteroidal antiinflammatory drugs (NSAIDs, such as ibuprofen or indomethacin), and aspirin
  • Antibiotics such as penicillin, sulfa drugs, and tetracycline
  • Antiseizure medications such as phenytoin (Dilantin) or carbamazepine (Tegretol)
• Risk factors for drug allergies include the following:
  • Frequent exposure to the drug
  • Large doses of the drug
  • Drug given by injection rather than pill
  • Family tendency to develop allergies and asthma
  • Certain food allergies such as to eggs, soybeans, or shellfish