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Drug Allergies

Condition Basics

What is a drug allergy?

A drug allergy occurs when your immune system overreacts to something in a medicine. It causes an allergic reaction. This response can range from mild symptoms to a severe whole-body reaction that can be deadly.

What causes it?

Any medicine can cause an allergic reaction. A few of the common ones are:

  • Penicillins (such as ampicillin or amoxicillin).
  • Sulfa medicines.
  • Anesthesia.
  • Aspirin and other nonsteroidal anti-inflammatory drugs.
  • Some medicines used to treat seizures.
  • Some medicines used to treat cancer.

If you are allergic to one medicine, you may be allergic to others like it. For example, if you are allergic to penicillin, there is a chance that you may also be allergic to similar medicines, such as amoxicillin.

What are the symptoms?

The symptoms of a drug allergy can range from mild to very serious, and can even cause death. Symptoms may appear within an hour or they could take days or weeks to appear. They include:

  • Hives or welts, a rash, and itchy skin.
  • Coughing, wheezing, a runny nose, and trouble breathing.
  • A fever.
  • Swelling of the throat, mouth, lips, or tongue.
  • Serious skin conditions that make your skin blister and peel. These include toxic epidermal necrolysis and Stevens-Johnson syndrome.
  • Anaphylaxis, which is the most dangerous reaction. It can be deadly, and you will need emergency treatment. Symptoms include hives all over your body, trouble breathing, belly pain, nausea or vomiting, swelling of the throat or mouth, and feeling very lightheaded. These usually appear within 1 hour after you take the medicine.

How is it diagnosed?

Your doctor will ask about the medicines you have taken. You'll be asked about your health and symptoms. Your doctor may do a physical exam. You may need skin tests or blood tests. Or your doctor may have you take small doses of a medicine to see if you have a reaction.

How is a drug allergy treated?

If you have severe drug allergies, your doctor may give you an epinephrine auto-injector. Inject epinephrine into the thigh muscle if you have signs of a severe allergic reaction, such as trouble breathing, hives all over your body, or fainting. Call 911 right away.

If you have a mild allergic reaction, over-the-counter (OTC) antihistamines may help your symptoms. You may need prescription medicine if OTC antihistamines don't help or if you have problems with side effects.

The best thing you can do for a drug allergy is to stop taking the medicine that causes it. Your doctor may give you another type of medicine instead.

If you can't change your medicine, your doctor may try giving you small amounts of the medicine that caused your reaction (desensitization therapy). Under your doctor's supervision, you will then slowly increase how much you take. This lets your immune system "get used to" the medicine. After this, you may no longer have an allergic reaction.

How can you care for yourself?

If you have a known drug allergy, there are some things you can do to help prevent a reaction.

  • Know which medicines you're allergic to, and avoid taking these medicines.
  • Keep a list of all medicines you are taking.
  • Talk with your doctor or pharmacist about any new medicines you are prescribed. Make sure they are not similar to those that can cause a reaction.
  • Be sure that anyone treating you for any health problem knows what medicines you are allergic to.
  • Don't use someone else's medicines or share yours.
  • If you are at risk for a severe allergic reaction to medicines, be sure to always have an epinephrine shot with you.

For a mild rash or itching, take a cool shower. Wear light clothes that don't bother your skin. Use calamine lotion. If you were given a medicine for your allergic reaction, take it as directed. If you have a severe reaction, use your epinephrine shot and call 911. Avoid medicines that cause your allergy.


Current as of: September 25, 2023

Author: Healthwise Staff
Clinical Review Board
All Healthwise education is reviewed by a team that includes physicians, nurses, advanced practitioners, registered dieticians, and other healthcare professionals.


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