Causes of Drug Allergy

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