Patient Handouts
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Drug Allergy
What is a drug allergy?
A drug allergy is a reaction by your immune system to a medicine you have taken. If you are allergic to a medicine, even a small amount can
trigger a reaction. The reaction can range from mild to life-threatening.
How does it occur?
When you have an allergic reaction to a medicine, your immune system treats the drug as a foreign substance and tries to protect you from
it. The medicines most likely to cause an allergic reaction are:
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antibiotics, such as penicillin
anti-seizure medicines
contrast dyes used for some X-rays
some heart and cancer drugs
local anesthetics (pain killers), such as lidocaine
What are the symptoms?
The most common symptoms are:
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itching
redness of the skin or a red rash
swelling of the skin
hives.
Symptoms of a drug allergy can happen within minutes after you take the first dose of a medicine. Or symptoms may start several days after
you start taking the medicine. About half of all allergic reactions happen 1 week after starting a drug. Most symptoms go away 3 to 5 days
after you stop taking the drug.
Anaphylaxis is a life-threatening allergic reaction. The reaction is sudden and severe and involves the whole body. Symptoms of a severe
reaction include:
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rash or hives
swelling of the lips, face, or throat
trouble breathing, often with wheezing
a hoarse voice
lightheadedness
fast or pounding heartbeat
nausea and vomiting
fainting.
How is it diagnosed?
Your healthcare provider will ask about your symptoms and the medicines you have taken. Tell your provider about all of the medicines you
are taking. This includes prescription and nonprescription drugs, supplements, natural remedies, herbs, and vitamins.
If you have a severe allergic reaction and your provider is not sure what caused the reaction, you may need to see an allergy specialist for
tests.
How is it treated?
A severe allergic reaction is life-threatening. If you think you are having a severe reaction, go to the emergency room or call 911 for help. A
severe or life-threatening reaction usually needs to be treated with:
Patient handouts provided by Merck Medicus. Copyright © 2012-2013 Merck Sharp & Dohme Corp., a subsidiary of Merck & Co., Inc.
Patient Handouts
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a shot of epinephrine (also called adrenaline) and
a shot of antihistamine, such as Benadryl (diphenhydramine)
Often a steroid medicine, such as hydrocortisone, is also needed.
You may be observed in the emergency room to make sure your treatment stops the allergic reaction. Usually you will be given medicine to
take after you go home to keep the reaction from happening again over the next several hours. Sometimes a reaction may be so severe that
you need to stay in the hospital for a while to make sure the swelling and your breathing and blood pressure all go back to normal.
A mild allergic reaction may be treated with just an antihistamine. You may also be given steroids as a shot or as a prescription to be taken
for the next few days. Using a steroid for a long time can have serious side effects. Take steroid medicine exactly as your healthcare provider
prescribes. Don't take more or less of it than prescribed by your provider and don't take it longer than prescribed. Don't stop taking a steroid
without your provider's approval. You may have to lower your dosage slowly before stopping it.
How long will the effects last?
The effects of an allergic reaction can last from several minutes to hours. How long it lasts depends on how much of the medicine you took
and how severe your reaction is.
How can I take care of myself?
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Follow your healthcare provider's instructions if you were given medicine to take at home.
If you had a severe reaction, your provider may prescribe an epinephrine emergency kit, such as EpiPen or Twinject. You will
need to always carry the kit with you. It contains a ready-to-use syringe of epinephrine. If you have a severe allergic reaction, a
shot of this medicine can counteract allergy symptoms until you get medical care. You or someone with you can give you the shot.
The kit is not intended as the sole treatment of an allergic reaction. Rather, it "buys" time while you wait for emergency help. You
should check the expiration date for this medicine and replace it as needed to make sure it will work.
Wear a bracelet or necklace that warns of your allergy and tells what to do in case of an emergency. Tell your family, friends, and
co-workers what they should do if you have a severe reaction.
How can I prevent allergic reactions?
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Avoid taking medicine that you have had an allergic reaction to. Be sure to check medicine labels for the names of drugs you are
allergic to before taking any medicine or natural remedy. A medicine may be available in different shapes and colors. Don't
depend on how a medicine looks to know whether it's the one you're allergic to.
Write down the name of any medicines you have reacted to and what your reaction was. Carry this information with you.
Tell all healthcare providers who treat you, including pharmacists and dentists, about all past allergic reactions you have had.
Patient handouts provided by Merck Medicus. Copyright © 2012-2013 Merck Sharp & Dohme Corp., a subsidiary of Merck & Co., Inc.