Drug Allergies

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.
• Stomach cramps, feeling sick to the stomach, vomiting, and diarrhea.
• 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.

Symptoms

Symptoms of drug allergies can be mild or life-threatening and appear within 1 to 72 hours. They include:

• Hives or welts, rash, swelling, redness, and blisters, or eczema. These are the most common symptoms of drug allergies, and they may occur right after you take a medicine or several days later. See an illustration of skin reactions caused by drug allergies.
• Coughing, wheezing, runny nose, skin color changes, and difficulty breathing.
• Stomach cramps, feeling sick to the stomach, vomiting, and diarrhea.
• 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.

A drug allergy can also affect the liver, kidneys, and lymph system. However, you usually do not have any symptoms in this case.

Medicines may also cause:

• Serum sickness. This rare condition usually begins 6 to 21 days after you take the offending medicine. Symptoms include fever, weakness and body aches, joint pain, and skin eruptions, such as hives or a rash.
• Medicine fever. Symptoms include high fever and chills and sometimes a skin rash. When you stop taking the medicine, the fever usually drops within 48 to 72 hours.
• Autoimmune disorders. Drug allergies can cause several autoimmune disorders, such as drug-induced lupus, vasculitis, and myasthenia gravis, although these are rare. See more information on medicines and lupus.
• Destruction of platelets and red blood cells, resulting in thrombocytopenia and hemolytic anemia, respectively. Symptoms of thrombocytopenia include bruising easily, red spots around the feet and ankles, and nosebleeds. Symptoms of hemolytic anemia include fever, chills, shortness of breath, and a rapid heart rate.

When to call a doctor

Call 911 or other emergency services right away if:

• You develop hives and have trouble breathing or other symptoms of anaphylaxis. If you have an epinephrine shot (such as EpiPen) available, give it to yourself while you have someone else call 911 .

See illustrations of hives and a skin rash caused by a drug allergy.

Call your doctor if:

• Your face, tongue, or lips are swollen, even if you are not having trouble breathing and the swelling is not getting worse.
• You develop a skin rash, itching, a feeling of warmth, or hives.
• Home treatment does not help and symptoms get worse.

Treatment

Treatment for drug allergies includes understanding what to do if you have a severe allergic reaction, avoiding the medicine that causes the allergy, and using medicines such as antihistamines for mild symptoms.

Emergency treatment

Call 911 or other emergency services immediately if:

• You develop hives and have trouble breathing or other symptoms of anaphylaxis. If you have an epinephrine shot (such as EpiPen) available, give it to yourself while you have someone else call 911 . See illustrations of hives and a skin rash caused by a drug allergy.

If you have a severe allergic reaction, your first treatment may occur in an emergency room, or emergency personnel may treat you where the reaction occurs. If you have not already given yourself an epinephrine shot, you may get one to help you breathe. You may also get antihistamines and corticosteroids.

Other treatment

You treat a drug allergy by avoiding the medicine that causes the allergic reaction. When your doctor knows which medicine is causing your reaction, he or she will find another medicine for you to use. If this is not possible, your doctor may suggest desensitization therapy.

In desensitization therapy, 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 to it. Because you may have a severe reaction during this therapy, it is done where emergency medical help is available and under the supervision of a health professional.

If you have a mild allergic reaction, several medicines may help ease symptoms. Prescription or over-the-counter antihistamine tablets or syrup, such as diphenhydramine hydrochloride (for example, Benadryl) or chlorpheniramine maleate (for example, Chlor-Trimeton), may lessen rash, itching, and other symptoms. Your doctor may also suggest corticosteroid medicine to decrease your symptoms. Take these medicines as your doctor tells you.

If you have severe drug allergies:

• Your doctor may give you an allergy kit that contains a shot of epinephrine and antihistamine tablets. Give yourself the epinephrine shot as soon as you feel a reaction starting. Then take the antihistamines.
• Go to the emergency room every time you have a reaction, even if you are feeling better. Symptoms can develop again even after the epinephrine shot.
• Keep your allergy kit with you at all times. Be sure to check the expiration dates on the medicines and replace them as needed.
• Always wear medical alert jewelry that lists your drug allergies. You can get this type of jewelry at most drugstores or on the Internet.

If you have allergies to many different medicines, be careful when you start any new medicine. The first time you take a new medicine, take it at your doctor's office or at a hospital where you can get immediate treatment if you have a reaction.