What causes a drug allergy?

An allergic reaction occurs when the body's immune system produces antibodies and activates disease-fighting cells in response to an offending substance in the environment – in this case, a specific drug.

The antibodies release chemicals that injure the surrounding cells and cause the physical symptoms of an allergic reaction. Certain specialized cells in the skin and mucous membranes release chemicals known as histamines.

In the first step of drug allergy development, the body becomes sensitized (i.e., the immune system is triggered) by the first exposure to the medication. A subsequent exposure causes an immune response, including the production of antibodies and release of histamine. Therefore, drug allergy reactions occur after a person has been previously exposed to the drug one or more times without any allergic reaction.

Drug allergies may be due to the direct effects of a medication, as occurs with penicillin, vaccines, blood transfusions, insulin and intravenous (IV) fluids. These medications involve the body's immune system directly.

Other drug allergies may be caused indirectly by medications that do not involve the immune system but trigger histamine release in the body. Medications such as ASA and anti-inflammatory drugs, morphine and related medications (known as opiates), local anesthetics, and some fluids given intravenously during X-rays can indirectly cause drug allergies.

In general, the number and severity of adverse reactions increase as the dose increases. However, this relationship doesn't apply to people who are allergic to a medication. For these people, even small amounts of the medication can trigger an allergic reaction, which can range from minor and simply annoying to severe and life threatening.

In theory, all medications can cause allergic drug reactions. Some common medications that have been associated with allergic reactions include:

  • antibiotics such as penicillin, cephalosporins, and sulphonamides
  • blood pressure medications such as ACE inhibitors
  • opiates such as codeine and morphine
  • non-steroidal anti-inflammatory drugs (ASA-like drugs) such as ibuprofen and indomethacin

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