When Alexander Fleming discovered penicillin in 1928, he probably didn’t predict some people would be allergic to it. But 10 percent of Americans think they are.

They may wrong.

According to the Centers for Disease Control and Prevention (CDC), less than 1 percent of the U.S. population has a true allergy to this group of antibiotics. Of those people who do, many stop being allergic to it over time. In fact, according to the CDC, about 80 percent of patients with a true (IgE-mediated) penicillin allergy lose their sensitivity after 10 years.

Here's why it's important to confirm whether or not you're allergic. If they can't give you penicillin (a "narrow-spectrum" antibiotic) for your bacterial infection, doctors will have to use a broad-spectrum antibiotic. That antibiotic may not work as well as penicillin depending on what kind of infection you have. Plus, the more doctors use broad-spectrum antibiotics, the more certain bacteria become antibiotic-resistant.

Related: Top Drug-Resistant Germs in America

How to find out

A doctor can figure out whether you have a true allergy to penicillin. He or she will probably start by getting your history, asking questions like:

  • What medication were you taking when you had the reaction?
  • What kind of reaction did you have?
  • How long ago did the reaction occur?
  • How was the reaction managed?
  • What was the outcome?

The doctor might give you a skin test to determine if you're allergic. According to the Mayo Clinic, a positive reaction will cause a red, itchy, raised bump.

If you have a positive reaction, you likely have a penicillin allergy. To confirm it, the doctor may follow up by giving you a bit of oral penicillin, perhaps a type that is less likely to trigger a reaction to see if you can tolerate it. This is known as an oral challenge. If you have no reaction, the doctor may give you increasingly larger doses. The doctor will watch to see how you respond and be ready to treat you if you have an anaphylactic reaction.

It's possible to be allergic to only certain kinds of penicillin, including penicillin G or benzylpenicillin, according to the CDC. An oral challenge can help identify this kind of allergy.

Related: 5 Common Antibiotic Mistakes

Penicillin allergy symptoms

You may be allergic to penicillin if these symptoms occur within one hour of taking the drug, according to the CDC:

  • Hives
  • Skin rash
  • Fever
  • Watery eyes
  • Wheezing and shortness of breath
  • Anaphylaxis

Related: These Prescription Drugs Are the Most Likely to Hurt You


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Muriel Vega is a writer with a passion for budget travel and staying safe while abroad. A Georgia State University graduate, she has over 6 years of editorial experience and has written for The Guardian, The Atlantic, The Billfold, among other outlets. In her free time, you can find her baking pies, playing with her two dogs and cat, or planning her next vacation. She spends way too much time on Twitter, one of her favorite social media channels. Her favorite safety tip: Make sure you have all the necessary shots before you go abroad.