Drug Allergies

If you develop a rash, hives or difficulty breathing after taking certain medications, you may have a drug allergy.

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As with other allergic reactions, these symptoms can occur when your body’s immune system becomes sensitized to a substance in the medication, perceives it as a foreign invader and releases chemicals to defend against it.

People with drug allergies may experience symptoms regardless of whether their medicine comes in liquid, pill or injectable form. If you feel confused about your symptoms and which medications are safe for you to take then it’s time to take control and see an allergist.

Drug Allergy Symptoms

  • Skin rash or hives
  • Itching
  • Wheezing or other breathing problems
  • Swelling
  • Anaphylaxis, a potentially life-threatening reaction that can simultaneously affect two or more organ systems (for example, when there is both a rash and difficulty breathing)

Reactions can occur in any part of your body.

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Common Triggers of Drug Allergies

  • Penicillin and related antibiotics
  • Antibiotics containing sulfonamides (sulfa drugs)
  • Anticonvulsants
  • Aspirin, ibuprofen and other nonsteroidal anti-inflammatory drugs (NSAIDs)
  • Chemotherapy drugs

Diagnosing Drug Allergies

  • Skin test (accurate only for penicillin)
  • Drug challenge

Drug Allergy Management and Treatment

  • Avoid triggers.
  • Seek immediate medical care if symptoms worsen or multiple symptoms occur together (anaphylaxis).
  • Make sure that all of your health care providers, including your pharmacist, are aware of your allergy. If you have a history of anaphylaxis, wear a medical alert bracelet that lists your trigger.


While you may not experience allergic symptoms the first time you take a drug, your body could be producing antibodies to it. As a result, the next time you take the drug, your immune system may see it as an invader, and you’ll develop symptoms as your body releases chemicals to defend against it.

These symptoms may include:

  • Skin rash or hives
  • Itching
  • Wheezing or other breathing problems
  • Swelling
  • Vomiting
  • Feeling dizzy or light-headed
  • Anaphylaxis, a potentially life-threatening reaction that can impair breathing and send the body into shock; reactions may simultaneously affect two or more organ systems (for example, when there is both a rash and difficulty breathing)

Penicillin causes most allergic drug symptoms. Just because you show allergic symptoms after taking penicillin doesn’t mean that you will react to related drugs, such as amoxicillin, but it’s more likely. Also, just because you had a reaction to penicillin (or any other drug) at one time doesn’t mean you will have the same reaction in the future.

Antibiotics that contain sulfa drugs, such as Septra and Bactrim (sulfamethoxazole-trimethoprim) and Pediazole (erythromycin-sulfisoxazole), occasionally cause allergic reactions. Nonantibiotic drugs containing sulfa are very low-risk.


Drug allergies can be hard to diagnose. An allergy to penicillin-type drugs is the only one that can be definitively diagnosed through a skin test. Some allergic reactions to drugs – particularly rashes, hives and asthma – can resemble certain diseases.

Your allergist will want to know the answers to these questions:

  • What drug do you suspect caused your reaction?
  • When did you start taking it, and have you stopped taking it?
  • How long after you took the drug did you notice symptoms, and what did you experience?
  • How long did your symptoms last, and what did you do to relieve them?
  • What other medications, both prescription and over-the-counter, do you take?
  • Do you consume herbal medications or take vitamin or mineral supplements? If so, which ones?

Your allergist will also want to know whether you have had a reaction to any other drug. If you can, bring the suspected drug with you. This will help the allergist recommend alternatives as needed.

During a physical examination, your allergist will look for problems that are part of the drug reaction, along with nonallergic reasons for the reaction

Depending on the drug suspected of causing the reaction, your allergist may suggest a skin test or, in limited instances, a blood test. A blood test may be helpful in diagnosing a severe delayed reaction, particularly if your physician is concerned that multiple organ systems may be involved. This rare reaction is known as “drug rash with eosinophilia and systemic symptoms” or, more commonly, “DRESS syndrome.”

If a drug allergy is suspected, your allergist may also recommend an oral drug challenge, in which you will be supervised by medical staff as you take the drug suspected of triggering a reaction. (If your reaction was severe, a drug challenge may be considered too dangerous.)

Management and Treatment

If you have a drug allergy:

  • Make sure all of your doctors are aware of your allergy and the symptoms you experienced.
  • Ask about related drugs that you should avoid.
  • Ask about alternatives to the drug that caused your allergic reaction.
  • Wear an emergency medical alert bracelet or necklace that identifies your allergy.


Anaphylaxis is a severe, potentially life-threatening reaction that can simultaneously affect two or more organ systems (for instance, when there is both swelling and difficulty breathing, or vomiting and hives). If this occurs, call 911 and seek emergency medical care immediately.

If you are caring for someone who appears to be having a severe reaction to a drug, tell the emergency care team what drug was taken, when it was taken and what the dosage was.

If your allergic reaction to a drug is not life-threatening, your allergist may give you:

  • An antihistamine to counteract the allergic reaction
  • A nonsteroidal anti-inflammatory drug, such as ibuprofen or aspirin, or a corticosteroid to reduce inflammation

Drug desensitization

If there is no suitable alternative to the antibiotic that you are allergic to, you will need to undergo drug desensitization. This involves taking the drug in increasing amounts until you can tolerate the needed dose with minimal side effects. This will most likely be done in a hospital so immediate medical care is available if problems develop.

Desensitization can help only if you are taking the drug every day. Once you stop it – for example, when a chemotherapy cycle ends – you will need to go through desensitization a second time if you need the drug again.

Penicillin Allergy

Nearly everyone knows someone who says they are allergic to penicillin. Up to 10% of people report being allergic to this widely used class of antibiotic, making it the most commonly reported drug allergy. That said, studies have shown that more than 90% of those who think they are allergic to penicillin, actually are not. In other words, 9 out of 10 people who think they have penicillin allergy are avoiding it for no reason. Even in people with documented allergy to penicillin, only about 20% are still allergic ten years after their initial allergic reaction.

Penicillin, famously discovered by Alexander Fleming in 1928, is prescribed today to treat a variety of conditions, such as strep throat. Despite its efficacy, some people steer clear of penicillin for fear of experiencing an allergic reaction to the medication.

Anyone who has been told they are penicillin allergic, but who hasn’t been tested by an allergist, should be tested. An allergist will work with you to find out if you’re truly allergic to penicillin, and to determine what your options are for treatment if you are. If you’re not, you’ll be able to use medications that are safer, often more effective and less expensive.

Penicillin Allergy Symptoms

Mild to moderate allergic reactions to penicillin are common, and symptoms may include any of the following:

  • Hives (raised, extremely itchy spots that come and go over a period of hours)
  • Tissue swelling under the skin, typically around the face (also known as angioedema)
  • Throat tightness
  • Wheezing
  • Coughing
  • Trouble breathing

A less common but more serious, sudden-onset allergic reaction to penicillin is anaphylaxis, which occurs in highly sensitive patients. Anaphylaxis occurs suddenly, can worsen quickly and can be deadly. Symptoms of anaphylaxis might include not only skin symptoms, but also any of the following:

  • Tightness in the chest and difficulty breathing
  • Swelling of the tongue, throat, nose and lips
  • Dizziness and fainting or loss of consciousness, which can lead to shock and heart failure

These symptoms require immediate attention at the nearest Emergency Room. Epinephrine, the therapy of choice, will be given in this urgent care setting, but should also be self-administered via autoinjector as soon as possible by patients who have already been prescribed and are wisely carrying this device.

Penicillin Allergy Testing and Diagnosis

An allergist is your best resource to help you evaluate the safety of taking penicillin.

In addition to assessing your detailed history about a prior allergic reaction to penicillin, allergists administer skin tests to determine if a person is or remains allergic to the medication. These tests, which are conducted in an office or a hospital setting, typically take about two to three hours, including the time needed after testing to watch for reactions.

When safely and properly administered, skin tests involve pricking the skin, injecting a weakened form of the drug, and observing the patients reaction. People who pass penicillin skin tests by reacting negatively to the injection are seen as at low risk for an immediate acute reaction to the medication. The allergist might then give these individuals a single, full-strength oral dose to confirm the absence of a penicillin allergy.

Those with positive allergy skin tests should avoid penicillin and be treated with a different antibiotic. If penicillin is recommended, people in some cases can undergo penicillin desensitization to enable them to receive the medication in a controlled manner under the care of an allergist.

Penicillin Allergy Treatment

Those who have severe reactions to penicillin should seek emergency care, which may include an epinephrine injection and treatment to maintain blood pressure and normal breathing.

Individuals who have milder reactions and suspect that an allergy to penicillin is the cause may be treated with antihistamines or, in some cases, oral or injected corticosteroids, depending on the reaction. Visit an allergist to determine the right course of treatment.

If you were told you are allergic to penicillin but have never been tested, it’s time to see an allergist.


The time varies from person to person. Some people may react right away, while others might take the drug several times before they have an allergic reaction. Most of the time symptoms will appear between 1-2 hours after taking the drug unless you have a more rare, delayed type reaction. Symptoms of these less common drug allergies include fever, blistering of the skin, and occasionally joint pain.

Symptoms from a drug allergy can be like other allergic reactions and can include hives or skin rash, itching, wheezing, light headedness or dizziness, vomiting and even anaphylaxis. A combination of these symptoms makes it much more likely that it is an allergy than nausea and vomiting on their own, which are common side effects of medications.

Like most other allergies, the primary drug allergy treatment is avoidance. Once you find out you are allergic to a certain drug, your allergist will advise you to stay away from that drug. You will need to tell all your health care providers so you aren’t prescribed the drug in any situation. It’s wise to wear a medical alert bracelet or necklace so medical personnel will know you are allergic. If you have a reaction to a drug, you need immediate treatment. The treatment will depend on how severe the reaction is. If a severe life-threatening reaction, called anaphylaxis, occurs, use your epinephrine auto injector and call 911.

Penicillin allergy symptoms vary, and can range from mild to severe. Symptoms may include hives, swelling – typically around the face, throat tightness, wheezing, coughing and trouble breathing.

Anaphylaxis is a less common, more serious life-threatening reaction. It can come on suddenly, worsen quickly and can be fatal. Symptoms can include the ones listed above and any of the following:

  • Itching, hives, red skin rash, flushing in combination with other symptoms such as:
    • Difficulty breathing and tightness in the chest
    • Swelling of the lips, throat, tongue and face
    • Dizziness and loss of consciousness or fainting, which could lead to shock

If you experience any of these more severe symptoms, immediately use your epinephrine auto-injector and call 911.

Although penicillin is a highly effective antibiotic, there are many others available if you have a penicillin allergy. Your doctor will decide which antibiotic to prescribe based on several factors including the type of bacteria, how severe the infection is, what your previous reaction was to penicillin and whether you’ve been tested for penicillin allergy.

Penicillin is the most common drug allergy. If you experience an allergic reaction after taking penicillin, you won’t necessarily have a similar reaction to related drugs such as amoxicillin. But it is more likely to happen. Anticonvulsant, aspirin, ibuprofen and chemotherapy drug allergies are also common. It’s possible to have a reaction to a drug at some point in your life and not suffer a similar reaction in the future.

Not necessarily. In fact, up to 80% of people will lose their penicillin allergy if they avoid the drug for 10 years. It is important to be tested by an allergist to determine if you still truly allergic

No, but for daily medications, desensitization only needs to be performed once by your allergist. If the medication is taken daily, your body remains desensitized. If more than 2 days pass between doses, your body may not “remember” the desensitized state, and desensitization may need to be performed again.

This page was reviewed for accuracy 2/28/2018.