Penicillin is one of the most widely prescribed antibiotics today. This family of medicines can treat bacterial infections like strep throat, pneumonia, and sinus and urinary infections. But penicillin can be dangerous for people who are allergic to it.

About 1 in 10 of all U.S. patients report a penicillin allergy, according to the Centers for Disease Control & Prevention. However, up to 90% of these patients don’t have a true allergy. And avoiding penicillin unnecessarily has its disadvantages. Summit Health allergist Adam Williams, MD, answers questions about penicillin allergy and how to find out if you are ­ – or still are – allergic.


What can happen if you’re allergic to penicillin?

An allergy is the body’s overreaction to normally harmless substances. Antibiotics like penicillin are among the most common substances to cause allergic reactions. A penicillin allergy can trigger:

  • Skin reactions such as itching, rashes, hives, swelling, and/or blisters
  • Respiratory symptoms such as wheezing and shortness of breath
  • Inflammation in other parts of the body including kidneys, lungs, and joints

Less commonly, a penicillin allergy can cause a severe reaction called anaphylaxis. Symptoms of anaphylaxis can be life-threatening and include rash, difficulty breathing, a dangerous drop in blood pressure, and loss of consciousness.

Most allergic reactions to penicillin happen within an hour of taking it. While anyone can have a penicillin allergy, your risk of allergy may be higher if you:

  • Previously experienced a drug allergy
  • Have had frequent exposure to penicillin


Why do some people report a penicillin allergy when they don’t have one?

Someone may believe they have a penicillin allergy for a few reasons:

  • They experience a side effect of the drug, such as nausea or headache, that’s mislabeled as an allergic reaction.
  • Signs and symptoms of an underlying bacterial or viral infection (including hives or other rashes) can also be mistaken as a penicillin allergy.
  • Reliable penicillin allergy tests weren’t available until more recently.

They may have had an actual allergic reaction to penicillin but it has been outgrown. Between 80% and 100% of patients ultimately test negative for penicillin allergy 10 years after their last positive test, according to the National Institutes of Health.


Is there a downside to avoiding penicillin if I’m not allergic to it?

Having an unconfirmed penicillin allergy can negatively impact your medical care. By avoiding penicillin unnecessarily, patients often receive alternative antibiotics that:

  • May have a higher risk of side effects
  • Increase the risk of developing antibiotic resistance
  • Are associated with higher healthcare costs
  • Can lead to other medical complications


How do I find out if I’m allergic to penicillin – or still allergic?

Talk to your provider if you’re unsure whether you have a true penicillin allergy. Better yet, ask to see an allergist for penicillin allergy evaluation .

Diagnosing a penicillin allergy may involve skin testing and/or an oral drug challenge. For a skin test, your provider will apply drops of penicillin on the skin and prick the surface then  inject a small, diluted amount just under the skin. If an itchy bump appears, the test is positive, and you likely have a penicillin allergy.

The allergist may also recommend  an oral drug challenge. For this test, you take penicillin by mouth under observation in the allergy office to confirm you can safely tolerate it.


What are my options if I have a true penicillin allergy?

Your provider will need to prescribe other antibiotics if you’re allergic to penicillin. But if you get an infection that only penicillin will treat, your provider may recommend desensitization.

Desensitization is a procedure that starts with tiny doses of penicillin, increasing them over time up to normal dosage under careful monitoring. Because desensitization is a temporary approach, the process must be repeated if penicillin is necessary for future treatment.


Summit Health can help.  

If you think you may have a penicillin allergy, make an appointment with an Dr. Williams at Summit Health. Tests are available and can determine if you still have an allergy.