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Causes
By Mayo Clinic staffA 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 creates antibodies to attack the medication. Chemicals released by these antibodies 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
The most common drug allergies are caused by penicillin, antibiotics closely related to penicillin and antibiotics that contain sulfonamides. Antibiotics can also cause nonallergic reactions such as a skin rash and digestive problems.
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 are common, but in most cases they aren't severe and symptoms improve quickly.
Nonallergic adverse reactions
In most 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, most often drug reactions are 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 NSAIDs. In some people, nonsteroidal anti-inflammatory drugs (NSAIDs), which include 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 swelling of the lips, tongue and face.
- Celik J. Drug allergy. In: Adkinson NF. Middleton's Allergy: Principles and Practice. 7th ed. Philadelphia, Pa.: Mosby; 2008. http://www.mdconsult.com/das/book/body/1461724933/0/1806/72.html?tocnode=56310767&fromURL=72.html#4-u1.0-B978-0-323-05659-5..00068-1_2330. Accessed Sept 12, 2009.
- Tips to remember: Adverse reactions to medications and drug allergy. American Academy of Allergy, Asthma & Immunology. http://www.aaaai.org/patients/publicedmat/tips/adversereactions.stm. Accessed Sept. 12, 2009.
- Li JL (expert opinion). Mayo Clinic, Rochester, Minn. Sept. 20, 2009.
- Montanaro A. Allergies to sulfonamide antibiotics and related drugs. http://www.uptodate.com/home/index.html. Accessed Nov. 3, 2009
- Siegrist, CA. Mechanisms underlying adverse reactions to vaccines. Journal of Comparative Pathology. 2007;137:S46.
- Adverse drug reactions. The Merck Manuals: Home Edition for Patients and Caregivers. http://www.merck.com/mmhe/sec02/ch015/ch015a.html. Accessed Sept 12, 2009.
