A drug allergy is the reaction of the immune system to a medicine. Any medicine — over-the-counter, prescription or herbal — can trigger a drug allergy. However, a drug allergy is more likely with certain medicines.

The most common symptoms of drug allergy are hives, rash and fever. But a drug allergy also may cause serious reactions. This includes a severe, life-threatening condition known as anaphylaxis.

A drug allergy is not the same as a medicine side effect. A side effect is a known possible reaction to a medicine. Side effects to medicines are listed on their labels. A drug allergy also is different from drug toxicity. Drug toxicity is caused by an overdose of medicine.

Symptoms of a serious drug allergy often happen within an hour after taking a medicine. Other reactions, particularly rashes, can happen hours, days or weeks later.

Drug allergy symptoms may include:

• Skin rash.
• Hives.
• Itching.
• Fever.
• Swelling.
• Shortness of breath.
• Wheezing.
• Runny nose.
• Itchy, watery eyes.

Anaphylaxis

Anaphylaxis is a rare, life-threatening drug allergy reaction that causes widespread changes in the way body systems function. Symptoms of anaphylaxis include:

• Tightening of the airways and throat, causing trouble breathing.
• Nausea or belly cramps.
• Vomiting or diarrhea.
• Dizziness or lightheadedness.
• Weak, fast pulse.
• Drop in blood pressure.
• Seizure.
• Loss of consciousness.

Other conditions resulting from drug allergy

Less common drug allergy reactions happen days or weeks after exposure to a medicine and may last for some time after you stop taking the medicine. These include:

• Serum sickness, which may cause fever, joint pain, rash, swelling and nausea.
• Drug-induced anemia, a reduction in red blood cells, which can cause fatigue, irregular heartbeats, shortness of breath and other symptoms.
• Drug rash with eosinophilia and systemic symptoms, also called (DRESS), which results in rash, high white blood cell count, general swelling, swollen lymph nodes and hepatitis infection that comes back after being dormant.
• Inflammation in the kidneys, also called nephritis, which can cause fever, blood in the urine, general swelling, confusion and other symptoms.

When to see a doctor

Call 911 or emergency medical help if you experience signs of a severe reaction or suspected anaphylaxis after taking a medicine.

If you have milder symptoms of a drug allergy, see a healthcare professional as soon as possible.

A drug allergy happens when the immune system mistakenly identifies a medicine as a harmful substance, such as a virus or bacterium. Once the immune system detects a medicine as a harmful substance, it develops an antibody specific to that medicine. This can happen the first time you take a medicine, but sometimes an allergy doesn't develop until there have been repeated exposures.

The next time you take the medicine, these specific antibodies flag the medicine and direct immune system attacks on the substance. Chemicals released by this activity cause the symptoms associated with an allergic reaction.

You may not be aware of your first exposure to a medicine, however. Some evidence suggests that trace amounts of a medicine in the food supply, such as an antibiotic, may be enough for the immune system to create an antibody to it.

Some allergic reactions may result from a somewhat different process. Researchers believe that some medicines can bind directly to a certain type of immune system white blood cell called a T cell. This event causes the release of chemicals that can result in an allergic reaction the first time you take the medicine.

Medicines commonly linked to allergies

Although any medicine can cause an allergic reaction, some medicines are more commonly associated with allergies. These include:

• Antibiotics, such as penicillin.
• Pain relievers, such as aspirin, ibuprofen (Advil, Motrin IB, others) and naproxen sodium (Aleve).
• Chemotherapy drugs for treating cancer.
• Medicines for autoimmune diseases, such as rheumatoid arthritis.

Nonallergic drug reactions

Sometimes a reaction to a medicine can produce symptoms that are virtually the same as those of a drug allergy. However, a drug reaction isn't triggered by immune system activity. This condition is called a nonallergic hypersensitivity reaction or pseudoallergic drug reaction.

Medicines that are more commonly associated with this condition include:

• Aspirin.
• Dyes used in imaging tests, known as radiocontrast media.
• Opiates for treating pain.
• Local anesthetics.

While anyone can have an allergic reaction to a drug, a few factors can increase someone's risk. These include:

• A history of other allergies, such as a food allergy or hay fever.
• A personal or family history of drug allergy.
• Increased exposure to a medicine because of high doses, repeated use or prolonged use.
• Certain infections commonly associated with allergic drug reactions, such as HIV infection or Epstein-Barr virus infection.

If you have a drug allergy, the best prevention is to avoid using the problem medicine. Steps you can take to protect yourself include the following:

• Inform healthcare professionals. Be sure that your drug allergy is clearly identified in your medical records. Inform other healthcare professionals, such as your dentist or any medical specialist.
• Wear a bracelet. Wear a medical alert bracelet that identifies your drug allergy. This information can ensure proper treatment in an emergency.

An accurate diagnosis is essential. Research has suggested that drug allergies may be overdiagnosed and that patients may report drug allergies that have never been confirmed. Misdiagnosed drug allergies may result in the use of less appropriate or more-expensive medicines.

A healthcare professional typically does a physical exam and asks you questions. Details about when symptoms started, the time you took the medicine, and improvement or worsening of symptoms are important clues for helping your health professional make a diagnosis.

Your health professional may order more tests or refer you to an allergy specialist, called an allergist, for tests. These may include the following.

Skin test

With a skin test, the allergist or a nurse administers a small amount of a suspect medicine to the skin with a tiny needle that scratches the skin, a shot or a patch. A positive reaction to a test often causes a red, itchy, raised bump.

A positive result suggests that you may have a drug allergy.

A negative result isn't as clear-cut. For some medicines, a negative test result usually means that you're not allergic to the medicine. For other medicines, a negative result may not completely rule out the possibility of a drug allergy.

Blood tests

A healthcare professional may order blood tests to rule out other conditions that could be causing symptoms.

While there are blood tests for detecting allergic reactions to a few medicines, these tests aren't used often because of the relatively limited research on their accuracy. They may be used if there's concern about a serious reaction to a skin test.

Results of diagnostic work-up

After looking at your symptoms and test results, a healthcare professional can usually reach one of the following conclusions:

• You have a drug allergy.
• You don't have a drug allergy.
• You may have a drug allergy — with varying degrees of certainty.

These conclusions can help when making future treatment decisions.

Treatments for a drug allergy can be divided into two general strategies:

• Treatment for present allergy symptoms.
• Treatment that may enable you to take an allergy-causing medicine if it's medically necessary.

Treating current symptoms

The following treatments may be used to treat an allergic reaction to a medicine:

• Stopping the medicine. If a healthcare professional determines that you have a drug allergy — or likely allergy — stopping the medicine is the first step in treatment. For many people, this may be the only intervention necessary.
• Antihistamines. Your health professional may prescribe an antihistamine or recommend a nonprescription antihistamine such as diphenhydramine (Benadryl). An antihistamine can block immune system chemicals triggered during an allergic reaction.
• Corticosteroids. Corticosteroids given as a shot or taken by mouth may be used to treat symptoms associated with more-serious reactions.
• Treatment of anaphylaxis. Anaphylaxis requires an immediate epinephrine shot. Hospital care also is needed to maintain blood pressure and support breathing.

Taking allergy-causing medicines

If you have a confirmed drug allergy, a healthcare professional likely would not prescribe the medicine that causes a reaction unless it is necessary. Sometimes — if the diagnosis of drug allergy is uncertain or there's no other treatment — your health professional may use one of two strategies to give you the suspect medicine.

With either strategy, your health professional provides careful supervision. Supportive care also is available in the event of an adverse reaction. These treatments are generally not used if medicines have caused serious, life-threatening reactions in the past.

Graded challenge

If the diagnosis of a drug allergy is uncertain and a healthcare professional judges that an allergy is unlikely, a graded drug challenge may be an option. With this procedure, you receive 2 to 5 doses of the medicine, starting with a small dose and increasing to the desired dose, also called the therapeutic dose.

If you reach the therapeutic dose with no reaction, then your health professional may recommend that you take the medicine as prescribed.

Drug desensitization

If it's necessary for you to take a medicine that has caused an allergic reaction, your care professional may recommend a treatment called drug desensitization. With this treatment, you receive a very small dose and then increasingly larger doses every 15 to 30 minutes over several hours or days. If you can reach the desired dose with no reaction, then you can continue the treatment.

See a healthcare professional if you have symptoms that may be related to a medicine you recently started taking or take regularly. Be prepared to answer the following questions. These details are important in helping determine the cause of your symptoms.

• What symptoms did you experience? Don't leave out details even if they seem not related.
• When did the symptoms start? Be as specific as possible.
• How long did the symptoms last?
• What new medicine have you taken?
• When did you take it?
• Have you stopped taking the new medicine?
• What other nonprescription or prescription medicines do you take?
• What herbal medicines, vitamins or other dietary supplements do you take?
• At what time of day do you take your other medicines or supplements?
• Have you increased the dosage of any regular medicine or supplement?
• Have you stopped taking your regular medicines or supplements?
• Did you take anything to treat your symptoms, and if so, what was the effect?
• Have you had a reaction to a medicine in the past? If so, what medicine was it?
• Do you have hay fever, food allergies or other allergies?
• Is there a history of drug allergies in your family?

You may want to take pictures of any condition, such as a rash or swelling, to show your healthcare team. These may help if symptoms have eased by the time of your appointment.