Overview

A drug allergy is the reaction of the immune system to a medicine. Any medicine — over-the-counter, prescription or herbal — can trigger a drug allergy. However, a drug allergy is more likely with certain medicines.

The most common symptoms of drug allergy are hives, rash and fever. But a drug allergy also may cause serious reactions. This includes a severe, life-threatening condition known as anaphylaxis.

A drug allergy is not the same as a medicine side effect. A side effect is a known possible reaction to a medicine. Side effects to medicines are listed on their labels. A drug allergy also is different from drug toxicity. Drug toxicity is caused by an overdose of medicine.

Symptoms

Symptoms of a serious drug allergy often happen within an hour after taking a medicine. Other reactions, particularly rashes, can happen 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

Anaphylaxis is a rare, life-threatening drug allergy reaction that causes widespread changes in the way body systems function. Symptoms of anaphylaxis include:

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

Other conditions resulting from drug allergy

Less common drug allergy reactions happen days or weeks after exposure to a medicine and may last for some time after you stop taking the medicine. These 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, also called (DRESS), which results in rash, high white blood cell count, general swelling, swollen lymph nodes and hepatitis infection that comes back after being dormant.
  • Inflammation in the kidneys, also called nephritis, which can cause fever, blood in the urine, general swelling, confusion and other symptoms.

When to see a doctor

Call 911 or emergency medical help if you experience signs of a severe reaction or suspected anaphylaxis after taking a medicine.

If you have milder symptoms of a drug allergy, see a healthcare professional as soon as possible.

Causes

A drug allergy happens when the immune system mistakenly identifies a medicine as a harmful substance, such as a virus or bacterium. Once the immune system detects a medicine as a harmful substance, it develops an antibody specific to that medicine. This can happen the first time you take a medicine, but sometimes an allergy doesn't develop until there have been repeated exposures.

The next time you take the medicine, these specific antibodies flag the medicine and direct immune system attacks on the substance. Chemicals released by this activity cause the symptoms associated with an allergic reaction.

You may not be aware of your first exposure to a medicine, however. Some evidence suggests that trace amounts of a medicine in the food supply, such as an antibiotic, may be enough for the immune system to create an antibody to it.

Some allergic reactions may result from a somewhat different process. Researchers believe that some medicines can bind directly to a certain type of immune system white blood cell called a T cell. This event causes the release of chemicals that can result in an allergic reaction the first time you take the medicine.

Medicines commonly linked to allergies

Although any medicine can cause an allergic reaction, some medicines are more commonly associated with allergies. These include:

  • Antibiotics, such as penicillin.
  • Pain relievers, such as aspirin, ibuprofen (Advil, Motrin IB, others) and naproxen sodium (Aleve).
  • Chemotherapy drugs for treating cancer.
  • Medicines for autoimmune diseases, such as rheumatoid arthritis.

Nonallergic drug reactions

Sometimes a reaction to a medicine can produce symptoms that are virtually the same as those of a drug allergy. However, a drug reaction isn't triggered by immune system activity. This condition is called a nonallergic hypersensitivity reaction or pseudoallergic drug reaction.

Medicines that are more commonly associated with this condition include:

  • Aspirin.
  • Dyes used in imaging tests, known as radiocontrast media.
  • Opiates for treating pain.
  • Local anesthetics.

Risk factors

While anyone can have an allergic reaction to a drug, a few factors can increase someone's risk. These include:

  • A history of other allergies, such as a food allergy or hay fever.
  • A personal or family history of drug allergy.
  • Increased exposure to a medicine because of high doses, repeated use or prolonged use.
  • Certain infections commonly associated with allergic drug reactions, such as HIV infection or Epstein-Barr virus infection.

Prevention

If you have a drug allergy, the best prevention is to avoid using the problem medicine. Steps you can take to protect yourself include the following:

  • Inform healthcare professionals. Be sure that your drug allergy is clearly identified in your medical records. Inform other healthcare professionals, such as your dentist or any medical specialist.
  • Wear a bracelet. Wear a medical alert bracelet that identifies your drug allergy. This information can ensure proper treatment in an emergency.
By Mayo Clinic Staff