A drug allergy occurs when your immune system mistakenly identifies a drug as if it were a harmful substance instead of a helpful remedy. Your immune system then reacts to the medication. Chemicals released by this reaction cause the signs and symptoms associated with an allergic reaction.

It isn't clear why some people develop drug allergies or other adverse drug reactions while others don't. Inherited traits may play a role, along with environmental factors and taking a number of medications over time.

Antibiotic allergy

Drug allergies are often caused by penicillin, antibiotics closely related to penicillin and antibiotics that contain sulfonamides. Antibiotics can also cause nonallergic reactions such as nausea or diarrhea.

Vaccine allergy

Rarely, allergic reactions occur after vaccination. In certain cases, allergic reactions may be caused by the vaccine itself, but more often an allergic reaction is triggered by other ingredients in the vaccine such as egg or neomycin. Nonallergic reactions to vaccines, such as redness and itching, are common, but in most cases they aren't severe and symptoms improve quickly.

Nonallergic adverse reactions

In many cases, what appears to be a drug allergy is actually a reaction that doesn't involve the immune system. Although they may seem like an allergy, drug reactions may be a drug side effect or signs of a drug sensitivity — not an allergic reaction.

Some examples of drugs that commonly cause nonallergic reactions include:

  • X-ray contrast. Some people are sensitive to intravenous (IV) contrast used in some X-ray tests. This reaction can cause itching, flushing and a drop in blood pressure.
  • Aspirin and other pain relivers. In some people, aspirin, ibuprofen (Advil, Motrin, others), naproxen (Aleve, Naprosyn, others) and others, can cause breathing trouble, wheezing and hives.
  • Antibiotics. Some antibiotics often cause reactions such as stomachache or diarrhea.
  • High blood pressure medication. Angiotensin-converting enzyme (ACE) inhibitors sometimes cause coughing and swelling of the lips, tongue and face.
Oct. 14, 2011

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