Think You’re Allergic to Penicillin? There’s a 90% Chance You’re Not

If you’ve had a penicillin allergy listed in your medical chart for years, perhaps since childhood, there’s a good chance it shouldn’t be there anymore. Up to 90% of people who believe they’re allergic to penicillin can take it safely. Nancy Wasserbauer Kingston, DO, an allergist at University Hospitals, explains why so many penicillin allergies are inaccurate, what the real risks are and how a simple test could help you take penicillin safely.

What Happens During an Allergic Reaction to Penicillin?

Not all reactions to penicillin are true allergic reactions, which typically occur within a couple of hours of taking the medication. Symptoms can include hives, swelling of the face, lips or throat, difficulty breathing, wheezing, or anaphylaxis in the most severe cases. These reactions happen when the immune system produces antibodies targeting penicillin.

A delayed reaction, which can occur at the end of a course of penicillin or even up to two weeks after, usually presents as a flat, mildly itchy or non-itchy rash. “We see that very commonly in children, and it can also occur in adults,” says Dr. Wasserbauer Kingston. “But a delayed rash is generally not a true allergic reaction and doesn’t mean you can’t receive penicillin in the future.”

A Simple Mislabeling

Penicillin is one of the most common allergies incorrectly listed in a person’s medical record, for several reasons.

• Allergies change over time. Roughly 80% of people lose their penicillin allergy within 10 years. The immune system stops producing the antibodies that caused the reaction and the allergy fades. That’s how someone labeled allergic as a child may have no allergy by the time they’re an adult, but the label stays in their chart.
• The reaction may not have been a true allergy. Parents often report a rash in children while taking penicillin. Many times, that rash is caused by the infection itself, not the medication. In addition, common side effects to antibiotics, such as nausea and diarrhea, may also be mistaken for allergic reactions. “Not all reactions to penicillin are truly allergic reactions, and just because your parents have a penicillin allergy doesn’t mean you have one too,” Dr. Wasserbauer Kingston says.
• Penicillin injections given decades ago were more likely to cause reactions than the pill forms used today. As a result, many people still have a penicillin allergy listed in their medical record based on a reaction that happened many years ago, when the medication and the way it was given were different.

Why the Label Matters, Even When You Feel Fine

A penicillin allergy can seem like a minor inconvenience, until you need an antibiotic. Penicillin and its related class of beta-lactam antibiotics are the first-line treatment for some of the most common and serious bacterial infections, including strep throat, sinus infections, pneumonia and, importantly for moms and babies, group B strep in pregnant women.

When a patient has an inaccurate penicillin allergy label and needs treatment, doctors must reach for second- or third-line treatment alternatives. The consequences of that are well-documented: patients on alternative antibiotics have higher rates of treatment failure, longer hospital stays, and increased mortality and morbidity from serious infections. Alternative antibiotics are also typically more expensive, and their broader use contributes to antibiotic resistance.

“If you’re misdiagnosed with a penicillin allergy and you require an antibiotic that’s not the first-line treatment, you will likely pay more for a medication that is less effective in treating your infection,” Dr. Wasserbauer Kingston says. “Removing that allergy from your chart opens up an entire class of antibiotics that can simplify and improve your care.”

A True Test: The PENFAST Score and Oral Challenge

In 2022, a new tool called the PENFAST score was introduced. It helps doctors determine whether someone is likely to have a true penicillin allergy. People at very low risk may be able to safely take a test dose of penicillin without needing other allergy testing first.

The score is calculated by measuring:

• How long ago the reaction occurred. Reactions more than five years ago score lower, reflecting the likelihood the allergy has faded.
• The severity and nature of the reaction. Anaphylaxis, significant swelling or severe immune reactions score higher; a mild rash at the end of treatment scores zero.
• Whether treatment was required. A reaction that resolved on its own scores lower than one requiring a doctor visit or emergency care.

A score of zero means the probability of a penicillin allergy is approximately 0.6 percent, which is low enough that a physician can often remove the allergy label from your medical record.

For patients whose PENFAST score is one, or for those with a lower score who want added reassurance, additional testing can be performed. The patient takes an oral dose of amoxicillin, a penicillin-class antibiotic, and is monitored for 60 to 90 minutes. If no reaction occurs, the allergy is removed from the medical record immediately.

True reactions during the challenge are rare, occurring in less than 1% of cases, and are typically limited to a mild rash. The clinical team is fully prepared to treat any reaction that does occur. The procedure is considered safe in pregnant patients as well.

“It is very unlikely that patients with low PENFAST scores will have a true allergy,” Dr. Wasserbauer Kingston notes.

Knowing Your Risk Before You’re Sick

Dr. Wasserbauer Kingston says it’s important to discuss your penicillin status with your doctor before you get sick. A penicillin allergy label is often inherited from parents who may or may not remember the circumstances of an illness and treatment, and yet they stay in medical records for decades without anyone questioning them.

An inaccurate label can be cleared in under two hours. This is especially important for pregnant women, because penicillin is the first-line treatment for group B strep, a common and potentially serious bacterial infection in pregnancy.

“Being proactive in reviewing a past penicillin allergy in your medical record — rather than waiting until you actually have an infection that needs treatment — is something we’re trying to draw awareness to,” she says. “A simple conversation with your doctor can help ensure you get the right care when you get sick.”

Related Links

Experts in allergies and immunology at University Hospitals specialize in drug allergies, including penicillin allergy evaluation.