Allergic to Penicillin? Actually, You May Not Be

January 23, 2019

Clinical Contributors to this Story

Kathleen K. Casey, M.D. contributes to topics such as Infectious Disease.

If you’ve been told you’re allergic to penicillin, you may want to double-check to see if that’s true. More than 32 million Americans have a penicillin allergy documented in their medical records, but about 95 percent of them can still take penicillin, a recent study found.

Many people who had a reaction to penicillin as children were documented as allergic when they actually were not. Others with an allergy may have outgrown it. People tend to think a reaction of any kind means they can never use that class of antibiotics again. But that’s not true, and avoiding this class of drugs when you need it could have a negative impact on your health.

Patients who believe they cannot take penicillin can take alternative antibiotics, but those medications are often inferior to penicillin or drugs in its class known as beta-lactams, explained Kathleen K. Casey, M.D.,  an Infectious Disease specialist with Hackensack Meridian Health Medical Group. Beta-lactam alternatives may take longer to work, target bacteria that don’t need to be destroyed or don’t work as well in all areas of the body. Some beta-lactam alternatives do not exist in pill form and must be administered intravenously.

“You only want to use an alternative if you need to,” Casey says.

Being mislabeled with a penicillin allergy can cause people to develop an antibiotic resistance and subsequent infections needlessly, Casey added. There are certain conditions where a patient would have the best outcome with penicillin or a beta-lactam, but does not get that treatment because of a misdiagnosis or by mislabeling themselves as allergic.

According to a recent study in JAMA, patients may experience a reaction when penicillin is given for a virus and not a bacterial infection, for instance. In that case, the reaction could be to the virus and not the penicillin. The “allergy” is then documented in a patient’s chart, and likely never questioned again.


What constitutes an actual penicillin allergy? The following are true penicillin allergy signs:

  • Hives
  • Swelling of the mouth
  • Shortness of breath or wheezing
  • Anaphylaxis

Patients can demonstrate other reactions that can be confused with allergies. Hives can be confused with rashes, for example. Children who had a rash or diarrhea after taking penicillin may not have a reaction to penicillin at all; in some cases they are experiencing side effects of the drug—that doesn’t mean it’s an allergy, Casey explained. Other antibiotics have a cross-reactivity with a virus that is misconstrued as having an allergic reaction.

Casey sees many adults who were told as children that they had an allergy, so they grew up telling their doctor they could not have penicillin when they may have been able to take the drug.

Too often, parents and doctors documented any reaction as an allergy, which has led to an epidemic of people misdiagnosed or mislabeled with penicillin allergies as evidenced in the study, Casey said.

Another reason why so many people avoid beta-lactam antibiotics is that they think the allergy is genetic and therefore avoided the drugs. But that’s not true, either.

“Penicillin allergies are not inheritable,” she notes.


In the age of superbugs and an increased call for antibiotic stewardship, knowing if you are or are not allergic to penicillin can go a long way to maintain optimal health, and best position you for when you do need an antibiotic.

If it’s been 10 years or longer since a patient has had a reaction, they may be able to take penicillin without any problems, Casey said.

Undergoing allergy testing is a simple way to find out if you have a true allergy to penicillin. If you do have an allergic reaction, a clinical team is right there to relieve it by using other medications.

Many patients are receptive to allergy testing when they realize that they may have been misdiagnosed or mislabeled themselves, and when they learn about why they should take penicillin if their body can tolerate it.

If you think you may have been misdiagnosed or mislabeled yourself, talk to your doctor.

“You’re limiting your options if you don’t get tested,” Casey adds.

Kathleen K. Casey, M.D., specializes in Infectious Disease and Internal Medicine. She practices in Neptune, NJ. To book an appointment, call 732-897-3995. To find a provider near you, visit

The material provided through Health Hub is intended to be used as general information only and should not replace the advice of your physician. Always consult your physician for individual care.

Annals of Allergy, Asthma and Immunology