Allergic to Penicillin? It’s Worth a Second Look.

By Hallie OBranovic, MSN, APRN, NP-C

The Truth About Penicillin Allergies

Penicillin allergy is the most widely studied antibiotic allergy in allergy research. Although roughly 10% of the population reports being allergic to penicillin, studies show up to 90% can tolerate the medication. Mislabeling individuals with a penicillin allergy is common. It can result in using alternative antibiotics that are often less effective and more expensive and may contribute to antibiotic resistance.

The good news? Your allergist can help determine if you are truly allergic and possibly remove penicillin from your allergy list.

What Does a Penicillin Allergy Look Like?

Penicillin has been used since its discovery in 1928, and by 1942, it was widely administered.

Although about 10% of people report a penicillin allergy, severe allergic reactions—such as anaphylaxis—are extremely rare, occurring in only 0.02% to 0.04% of cases. Most reported reactions are mild, usually involving skin symptoms like rashes.

Penicillin allergies are linked to IgE antibodies, proteins the immune system produces in response to allergens. These antibodies decrease over time. For individuals who previously tested positive for a penicillin allergy, the chances of having another reaction drop by about 10% each year. This means that many people may outgrow their allergy, and retesting can help confirm if it’s safe for them to retake penicillin.

What to Expect During a Penicillin Allergy Evaluation

When we see someone with a penicillin allergy in our clinic, we take a detailed history of your reaction. You can expect questions like:

When did the reaction happen, and how long did it last?
What dose of the medication were you taking?
Why were you taking the medication (infection type, etc.)?
How did you take the medication (oral, intravenous, or other)?
How far into the course of treatment did the reaction occur (first dose, later doses)?
What other factors might have contributed (other medications, illness, diet changes, etc.)?
What symptoms did you experience during the reaction (rash, swelling, breathing issues, etc.)?
Was any treatment given at the time, and did it help?

Evaluating medication reactions is an essential part of the allergy evaluation process. Sometimes, patients tell us, “I’ve always been told I’m allergic,” without knowing the specifics. That’s okay! We can still help. In these cases, we will discuss options for testing or a medication challenge. Typically, this involves an initial consultation to review your history and create a personalized plan. Once that’s in place, you’ll return for testing or a challenge in a controlled, supervised setting.

Penicillin Testing and Challenge Process

At Boulder Medical Center, when you come into our Louisville, CO clinic for allergy testing, here’s what you can expect:

First, we will check your vital signs and ensure that you have not been sick in the week leading up to your visit. It’s essential that you feel well on the day of your test because illness can affect how your immune system responds, which could impact the accuracy of the results.
We will also ask that you stop taking antihistamines for at least five days before your appointment.
On the day of testing, it’s best to eat a light meal beforehand since antibiotics can sometimes cause an upset stomach. This helps us avoid confusing a normal side effect with a genuine allergic reaction.

The testing process depends on your history. If we need to start with skin testing, we’ll begin with a simple scratch test using a small plastic-tipped device on the surface of your skin. This sits for about 15 minutes before we check for any reaction. If that test is negative, we’ll move on to intradermal testing, which involves injecting a tiny amount of the medication just under the skin. We’ll wait another 15 minutes before taking another reading and move on to the oral challenge if all skin tests are negative.

In some cases, we skip skin testing and proceed directly to the oral challenge. This may involve taking a single full dose or gradually increasing the dose over time. You’ll be with us for 2–3 hours, so we can carefully monitor you for any reactions. We recommend bringing something to read, watch, or work on to help pass the time.

Why Re-Evaluating Your Allergy Matters

Accurate diagnosis of penicillin allergy is essential for antibiotic stewardship, which aims to ensure that patients receive the most effective treatment for bacterial infections. Unnecessary avoidance of penicillin can limit your options and contribute to antibiotic resistance.

Working with your allergist can help you better understand your allergy status and access the proper medications when needed.

Ready to Take the Next Step?

If you think you’ve been incorrectly labeled with a penicillin allergy or have questions about other medication allergies, consider seeing our allergy team for evaluation. Understanding your allergy status could make a big difference in your future care.

Helpful Resources

References

Patterson RA, Stankewicz HA. Penicillin Allergy. [Updated 2023 Jun 20]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan. Available from: https://www.ncbi.nlm.nih.gov/books/NBK459320/

Hallie Obranovic, MSN, APRN, NP-C

About Hallie OBranovic, MSN, APRN, NP-C
Allergy and Immunology at Boulder Medical Center

Get to know Hallie and make an appointment

As a practitioner in Boulder Medical Center’s Louisville allergy clinic, Hallie OBranovic brings a passion for personalized, patient-centered care and extensive experience in allergy and immunology. She began her medical career in pediatric intensive care, where she developed exceptional communication skills with families. Transitioning into family medicine and later specializing in allergy and immunology, Hallie discovered her passion for helping patients manage complex conditions. She describes her work in allergy and immunology as challenging and deeply rewarding.

Hallie provides care for a wide range of allergy and immunology concerns, including:

Allergic rhinitis
Asthma
Atopic dermatitis
Food allergies
Anaphylaxis
Hives
Drug allergies
Sinusitis
Immune deficiencies
Oral food and medication challenges
Hallie enjoys collaborating with families to solve complex cases

Allergic to Penicillin? You May Not Be.