Drug allergy

A drug allergy is the abnormal reaction of your immune system to a medication. Any medication — over-the-counter, prescription or herbal — is capable of inducing a drug allergy. However, a drug allergy is more likely with certain medications.

The most common signs and symptoms of drug allergy are hives, rash or fever. A drug allergy may cause serious reactions, including anaphylaxis, a life-threatening condition that affects multiple body systems.

A drug allergy is not the same as drug side effects, the known possible reactions that are listed on a drug label. A drug allergy is also distinct from drug toxicity caused by an overdose of medication.

Symptoms Causes Risk factors Prevention

Signs and symptoms of a drug allergy often occur within an hour after taking a drug. Less commonly, reactions can occur hours, days or weeks later.

Drug allergy symptoms may include:

  • Skin rash
  • Hives
  • Itching
  • Fever
  • Swelling
  • Shortness of breath
  • Wheezing
  • Runny nose
  • Itchy, watery eyes


Anaphylaxis is a rare, life-threatening reaction to a drug allergy that causes the widespread dysfunction of body systems. Signs and symptoms of anaphylaxis include:

  • Tightening of the airways and throat, causing trouble breathing
  • Nausea or abdominal cramps
  • Vomiting or diarrhea
  • Dizziness or lightheadedness
  • Weak, rapid pulse
  • Drop in blood pressure
  • Seizure
  • Loss of consciousness

Other conditions resulting from drug allergy

Less common drug allergy reactions occur days or weeks after exposure to a drug and may persist for some time after you stop taking the drug. These conditions include:

  • Serum sickness, which may cause fever, joint pain, rash, swelling and nausea
  • Drug-induced anemia, a reduction in red blood cells, which can cause fatigue, irregular heartbeats, shortness of breath and other symptoms
  • Drug rash with eosinophilia and systemic symptoms (DRESS), which results in rash, high white blood cell counts, general swelling, swollen lymph nodes and recurrence of dormant hepatitis infection
  • Inflammation in the kidneys (nephritis), which can cause fever, blood in the urine, general swelling, confusion and other symptoms

When to see a doctor

See your doctor as soon as possible if you experience signs or symptoms of drug allergy.

Call 911 if you experience signs of a severe reaction or suspected anaphylaxis after taking a medication.

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