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


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.


  • 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 is 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, Bactrim, and Pediazole, occasionally cause allergic reactions. Non-antibiotic 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 whether you have had a reaction to any other drug. If you can, bring the suspected drug with you. This will help your allergist recommend alternatives as needed.

Depending on the drug suspected of causing the reaction, your allergist may suggest a skin test.

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.)


If you have a drug allergy:

  • Make sure all your doctors are aware of your allergy and the symptoms you experienced.

  • Ask about related drugs 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 (http://acaai.org/allergies/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 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 non-steroidal anti-inflammatory drug, such as ibuprofen or aspirin, or a corticosteroid to reduce inflammation


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.


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. Over time however, the vast majority of people who once had a severe allergic reaction to penicillin loses sensitivity and can be treated safely with the drug (although a small percentage of individuals will remain allergic).


When properly administered, skin tests involve pricking the skin, injecting a weakened form of the drug, and observing the patient’s reaction. People who pass penicillin skin tests by reacting negatively are then scheduled for a graduated oral challenge with a penicillin class drug on another day. The oral challenge involves gradual administration of diluted solutions of penicillin orally with careful, timed observation until full-strength is administered. Usually the antibiotic is continued at home for several days to assess for delayed reactions.