Drug Allergies

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Topic Overview

What is a drug allergy?

A drug allergy occurs when your immune system overreacts to something in a medicine. This causes an allergic reaction. Although drug allergies can be life-threatening, most of them are mild, and symptoms go away within a few days after you stop using the medicine.

Although drug allergies may go away with time, once you have an allergic reaction to a drug, you will usually always be allergic to that drug or to others that are like it.

A drug allergy is a type of harmful, or adverse, drug reaction (such as a side effect or a reaction from taking more than one type of medicine). Because symptoms and treatments vary, it is important for your doctor to find out whether you have a drug allergy or another type of adverse reaction. Adverse reactions are usually not serious.

What are the symptoms of a drug allergy?

The symptoms of a drug allergy vary from mild to life-threatening and include:

• Hives or welts, rash, blisters, or eczema. These are the most common symptoms of drug allergies. See an illustration of skin reactions caused by drug allergies .
• Coughing, wheezing, runny nose, and difficulty breathing.
• Fever.
• Toxic Epidermal Necrolysis, is a serious life-threatening condition that involves blistering, and peeling of the skin.
• Anaphylaxis, which is the most serious reaction. It is life-threatening, and you will need emergency treatment. Symptoms usually appear within 1 hour after you take the medicine and include hives, difficulty breathing, and shock.

What medicines commonly cause an allergic reaction?

Any medicine can cause an allergic reaction. However, penicillins (such as nafcillin, ampicillin, and amoxicillin) cause most drug allergies. Sulfa medicines (sulfonamides), barbiturates, insulin, vaccines, anticonvulsants, and antithyroid medications for hyperthyroidism also can cause drug allergies.

If you are allergic to one medicine, you may be allergic to a similar medicine. For example, if you are allergic to penicillin, you may also be allergic to similar antibiotics such as cephalosporins (cephalexin or cefuroxime, for example).

People with AIDS or lupus may be allergic to many types of medicines. While the reactions are usually not life-threatening, they make treating the disease more difficult.

Some people—especially those with asthma—have allergy-like reactions to common pain relievers such as aspirin and ibuprofen. These reactions are not considered allergic reactions, because they do not affect the immune system. However, these reactions can be severe in those with asthma.

How is a drug allergy diagnosed?

Your doctor will diagnose a drug allergy by asking you questions about the medicines you are or have recently been taking, your past health, and your symptoms, and by doing a physical exam.

If your doctor cannot tell whether you have a drug allergy from doing this, he or she may do skin tests or have you take small doses of a suspected medicine to see whether a reaction occurs. Sometimes you will have a blood test or other type of testing.

How is a drug allergy treated?

The best thing you can do for a drug allergy is avoid the medicine that causes it and know what to do if you have an allergic reaction.

• Your doctor may be able to give you another type of medicine.
• If you have a life-threatening allergic reaction, you may need to give yourself an epinephrine shot and seek emergency medical treatment. You may also need to take other medicines, such as antihistamines and corticosteroids, directly into your vein (intravenously).
• If you have a mild allergic reaction, over-the-counter antihistamines may help your symptoms. You may need prescription medicine if these do not help or if you have problems with side effects, such as drowsiness.

If you cannot change your medicine, your doctor may try desensitization, in which you start taking small amounts of the offending medicine and gradually increase how much you take. This lets your immune system "get used to" the medicine, and you may no longer have an allergic reaction.

You should wear a medical alert bracelet or other jewelry that lists your drug allergies.

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Last updated:	July 24, 2007
Author:	Caroline Rea, RN, BS, MS
Reviewed By:	Caroline S. Rhoads, MD - Internal Medicine, Harold S. Nelson, MD - Allergy and Immunology
Editors:	Susan Van Houten, RN, BSN, MBA, Pat Truman