What is a drug allergy?

A drug allergy is a form of bad physical reaction to a medication. There are different forms of allergic reactions to drugs, including immediate and delayed hypersensitivity reactions.

Immediate reactions (anaphylaxis) occur when the drug enters the body. The medication triggers a response by the immune system, and creates specific IgE antibodies (proteins made by the immune system to fight the drug). This is called “sensitization.”

When the medication is taken again, the IgE antibodies release large amounts of the chemical histamine, which try to rid the drug from the body. During a delayed reaction, immune cells help to fight the drug.

Which drugs most often cause an allergic reaction?

The most common cause of drug allergies is penicillin and other antibiotics similar to penicillin. Other drugs that can cause reactions (that do not involve IgE antibodies) include:

  • Sulfa drugs
  • Anticonvulsants
  • Non-steroidal agents (like aspirin and ibuprofen)
  • Contrast dye
  • Chemotherapy drugs

What are the symptoms of drug allergy?

Symptoms of allergic reactions can range from mild itching to life-threatening conditions. Many drugs can also cause side effects or intolerances such as an upset stomach. These symptoms do not always indicate a true allergy to a drug.

During an allergic reaction, histamine and other chemicals can cause symptoms that include:

Symptoms of more serious reactions include:

  • Difficulty breathing
  • Blueness of the skin
  • Dizziness
  • Fainting
  • Drop in blood pressure

Last reviewed by a Cleveland Clinic medical professional on 06/15/2016.


  • American College of Allergy, Asthma and Immunology. Drug Allergies Accessed 6/15/2016.
  • American Academy of Allergy, Asthma and Immunology. Drug Allergy Accessed 6/15/2016.
  • Covar RA, Fleischer DM, Boguniewicz M. Chapter 38. Allergic Disorders. In: Hay, Jr. WW, Levin MJ, Deterding RR, Ross JJ, Sondheimer JM, eds.CURRENT Diagnosis & Treatment: Pediatrics. 21st ed. New York: McGraw-Hill; 2012.
  • Arroliga ME, Pien L. Penicillin allergy: consider trying penicillin again. Cleve Clin J Med 2003; 70: 313–8.

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