Do You Have a True Penicillin Allergy?

For years, I used to think I was allergic to penicillin. I made sure I told new doctors this so they could write it on my medical records. Then one day a new doctor questioned whether I had a true penicillin allergy or just a reaction to this antibiotic. It turns out, I had been wrong about my diagnosis all along.

The difference between a reaction and an allergy

When we talk about an allergy, we are referring to the immune system kicking in and overreacting to an antigen or allergen. A reaction is more like a side effect and does not necessarily mean your immune system is affected or overstimulated.

Do you have the symptoms of a true penicillin allergy?

This would include would hives, rash, swollen lips and mouth, and itchy skin. More serious symptoms include difficulty breathing, drop in blood pressure, dizziness, and loss of consciousness as part of a condition called anaphylaxis. If you do not experience any of these symptoms while taking penicillin, you are most likely not allergic to it.

Did you have these symptoms after a second dose?

Sometimes you do not have a reaction to any allergen until you receive a second dose of it. The reason being is that the body can experience what is called a sensitizing dose. The sensitizing dose refers to the first time you are exposed to an allergen. This makes your body sensitive to the allergen but does not give you any signs or symptoms with initial exposure. It’s only when you are exposed to this allergen a second time that your immune system reacts.

How about diarrhea and nausea?

Diarrhea and nausea are typical reactions to penicillin, and antibiotics in general, which makes people think they are allergic. This was what I experienced and caused me to believe I had a true penicillin allergy.

The odds are against you

According to the Journal of the American Medical Association, less than 1% of all people have a true penicillin allergy. Reactions, however, occur in about 10% of all those take it.

Do I have a true penicillin allergy?

If you have experienced some of the above symptoms after taking drugs like amoxicillin or ampicillin, you may have a true penicillin allergy. If you have never taken penicillin and want to know if you are allergic to it, speak with your doctor about a diagnostic skin test.


Mayo Clinic, Penicillin Allergy-Symptoms, December 2009

JAMA Evidence, Make the Diagnosis-Penicillin Allergy, Sensitizing Dose and Penicillin

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