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Many allergic reactions start within minutes of taking a drug. However, it's possible to develop an allergic reaction to a medication after you've been on it for up to several weeks.

Drug allergy symptoms include:

• Skin rash
• Hives (urticaria)
• Itching
• Fever
• Facial swelling
• Shortness of breath
• Anaphylaxis, a life-threatening reaction

Anaphylaxis
Anaphylaxis is rare, but it is the most serious drug allergy reaction and is a medical emergency. Anaphylaxis symptoms usually start within minutes after exposure to a drug. Signs and symptoms of anaphylaxis include:

• Tightening (constriction) of the airways and throat, causing trouble breathing
• Shock, with a severe drop in blood pressure
• Weak, rapid pulse
• Nausea, vomiting or diarrhea
• Dizziness, lightheadedness or loss of consciousness

It's possible to have an allergic response to a drug that caused no problem in the past.

If you have an anaphylactic reaction to a drug, your immune system responds to the drug as a harmful invader. This causes the release of histamine and other chemicals that cause allergic symptoms. Your immune system then becomes keyed to react the same way if you take the drug again in the future. However, the immune system changes over time, and eventually it's possible your drug allergy may go away on its own.

When to see a doctor
Talk to your doctor if you have a reaction to a drug or if you have any signs or symptoms of a drug allergy.

Call your doctor if you have a reaction after you take a drug. Mild allergic reactions are usually treated by stopping the drug and substituting another. If possible, see your doctor when the allergic reaction is occurring. This will help identify the cause and make sure you get treatment if it's needed.

Seek emergency treatment for signs of a severe reaction or suspected anaphylaxis after taking a medication. Signs and symptoms of an emergency drug reaction include:

• Swelling or tightening of the airways or throat
• Rapid pulse
• Dizziness
• Lightheadedness
• Loss of consciousness

©1998-2012 Mayo Foundation for Medical Education and Research (MFMER). Terms of use.
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A drug allergy occurs when your immune system mistakenly identifies a drug as if it were a harmful substance instead of a helpful remedy. Your immune system then reacts to the medication. Chemicals released by this reaction cause the signs and symptoms associated with an allergic reaction.

It isn't clear why some people develop drug allergies or other adverse drug reactions while others don't. Inherited traits may play a role, along with environmental factors and taking a number of medications over time.

Antibiotic allergy
Drug allergies are often caused by penicillin, antibiotics closely related to penicillin and antibiotics that contain sulfonamides. Antibiotics can also cause nonallergic reactions such as nausea or diarrhea.

Vaccine allergy
Rarely, allergic reactions occur after vaccination. In certain cases, allergic reactions may be caused by the vaccine itself, but more often an allergic reaction is triggered by other ingredients in the vaccine such as egg or neomycin. Nonallergic reactions to vaccines, such as redness and itching, are common, but in most cases they aren't severe and symptoms improve quickly.

Nonallergic adverse reactions
In many cases, what appears to be a drug allergy is actually a reaction that doesn't involve the immune system. Although they may seem like an allergy, drug reactions may be a drug side effect or signs of a drug sensitivity — not an allergic reaction.

Some examples of drugs that commonly cause nonallergic reactions include:

• X-ray contrast. Some people are sensitive to intravenous (IV) contrast used in some X-ray tests. This reaction can cause itching, flushing and a drop in blood pressure.
• Aspirin and other pain relivers. In some people, aspirin, ibuprofen (Advil, Motrin, others), naproxen (Aleve, Naprosyn, others) and others, can cause breathing trouble, wheezing and hives.
• Antibiotics. Some antibiotics often cause reactions such as stomachache or diarrhea.
• High blood pressure medication. Angiotensin-converting enzyme (ACE) inhibitors sometimes cause coughing and swelling of the lips, tongue and face.

©1998-2012 Mayo Foundation for Medical Education and Research (MFMER). Terms of use.
Read this article on Mayoclinic.com.

While anyone can have an allergic or nonallergic reaction to a drug, a few factors can increase your risk. These include:

• Having a past allergic reaction to the same drug or another drug. Even if past reactions have been mild, you may still be at risk of a more severe reaction.
• Taking a similar drug to one that caused a reaction in the past. For example, if you've had a reaction to penicillin, you also may be sensitive to some other antibiotics.
• Having a weakened immune system from conditions such as the Epstein-Barr virus or HIV/AIDS.
• Having a history of other allergies, such as hay fever.
• Taking several drugs at the same time, or taking frequent doses of the same medication.

Those who have a greater risk of developing a more severe reaction to medications include people with:

• Asthma
• Heart disease
• High blood pressure

©1998-2012 Mayo Foundation for Medical Education and Research (MFMER). Terms of use.
Read this article on Mayoclinic.com.

Complications of serious drug reactions can include:

• Anaphylaxis. This severe allergic reaction can be life-threatening and is a medical emergency.
• Drug-induced anemia. This occurs when a drug causes an immune system reaction that destroys blood cells.
• Serum sickness. Serum sickness can cause serious symptoms and lead to organ damage. Signs and symptoms include rash and joint pain, usually starting a week or so after you begin taking a drug.

Drug reactions can result in:

• Taking a less effective medication. If you have an allergic or nonallergic adverse reaction to a medication, you may need to switch to a medication that may not work as well.
• The need to use a drug that causes a reaction. If there's no other drug that works, you may need to use a drug that causes long-term, bothersome side effects.

©1998-2012 Mayo Foundation for Medical Education and Research (MFMER). Terms of use.
Read this article on Mayoclinic.com.

If you have a reaction to a drug, see your doctor as soon as possible. If the reaction is severe, get emergency help. If you had a past reaction and are now following up with your doctor, here are some things you can do to make the most of your appointment.

• Write down any signs and symptoms before you go to a doctor's appointment.
• Write down any treatments you tried at home, and let your doctor know whether or not they were successful.
• Make a list of any medications, herbal supplements or vitamins that you were taking when you had the reaction. Be sure to include any medications you put on your skin (topical medications) on your list.
• Tell your doctor about any health conditions you have, especially about any allergies.

What to expect from your doctor
Your doctor is likely to ask you a number of questions. Being ready to answer them may reserve time to go over any points you want to spend more time on. Your doctor may ask:

• When did you begin having symptoms?
• Do you only have these symptoms after taking a certain drug?
• Do you have this reaction every time you take this medication?
• How severe are your symptoms?
• Have you had a reaction to a drug in the past? If so, what drug was it?
• Do you have hay fever or any other allergies?
• What medications are you taking; when did you start taking them?
• Do you take any vitamins, supplements or herbal remedies?

©1998-2012 Mayo Foundation for Medical Education and Research (MFMER). Terms of use.
Read this article on Mayoclinic.com.

If you have an unexpected drug reaction, steps that your doctor may take to diagnose the source of the problem include:

• A physical examination. In addition to a physical exam, your doctor will have a number of questions. For example, he or she might ask how long after you took the medication you began having symptoms.
• Blood tests. A blood sample can only be used to detect an allergy to a few drugs, such as certain antibiotics, muscle relaxants and insulin. Skin tests are generally preferred because they're more accurate in detecting drug allergy, but blood tests may be used when someone has had a severe reaction in the past, because skin testing could cause another such reaction.
• Skin tests. For some drugs, including certain antibiotics, an allergy skin test may be used to determine whether you're allergic. A small amount of the drug is injected into the skin of your forearm or back. If you're allergic to the drug being tested, you develop a red, raised bump or other reaction.
• Drug provocation tests. During drug provocation testing, gradually increasing doses of the offending drug are given. The drug can be given in different ways for the test, including orally or under the skin. A reaction indicates a possible allergy or sensitivity to the drug. If reactions to the drug are mild or there's no evidence of an allergic reaction, the drug may be a safe treatment choice. This test is usually used only when an alternative drug won't work as well or isn't an option. It may also be used when skin and blood tests aren't conclusive. Risks include a severe reaction and possibly anaphylaxis. Generally, only specialized allergy centers perform provocation tests for drug allergy.

©1998-2012 Mayo Foundation for Medical Education and Research (MFMER). Terms of use.
Read this article on Mayoclinic.com.

Drug allergy treatment generally involves stopping the medication. You may also need medications to ease symptoms or, in the case of a serious reaction, emergency care.

• Minor reactions such as rashes or hives may improve with an over-the-counter antihistamine such as diphenhydramine (Benadryl, others). Call your doctor before using over-the-counter medications to make sure you're getting the treatment you need.
• Serious reactions may require treatment with oral or injected corticosteroids at the hospital. Seek emergency treatment if you have severe rashes or hives, swelling, shortness of breath, dizziness, or other signs or symptoms of a severe reaction.
• Anaphylaxis is an emergency requiring an immediate epinephrine injection and hospital care to maintain blood pressure and support breathing. If you've already had a serious allergic drug reaction, ask your doctor whether you need to carry an epinephrine injection (EpiPen, EpiPen Jr, Twinject).

Desensitization
In some cases, sensitivity to a drug can be reduced by starting with a tiny dose and gradually increasing it over time. This is done with medical supervision at a doctor's office, hospital or allergy clinic. In general, this is done only when you're allergic to a drug and a satisfactory alternative isn't available.

©1998-2012 Mayo Foundation for Medical Education and Research (MFMER). Terms of use.
Read this article on Mayoclinic.com.

If you have a history of a possible drug allergy, a skin test may help find out for certain. Tests for penicillin allergy are generally more reliable than are skin tests for allergies to other drugs.

Once you know you have a drug allergy, you'll need to avoid that drug and related drugs. Tell all of your health care providers, including your dentist, about your drug allergy. In case you're in an accident, you may want to wear a medical alert ID bracelet so that emergency workers will know about your allergy. In addition, you may want to carry a portable epinephrine injection (EpiPen, EpiPen Jr, Twinject) with you.

©1998-2012 Mayo Foundation for Medical Education and Research (MFMER). Terms of use.
Read this article on Mayoclinic.com.