Most people with penicillin allergy can tolerate cephalosporin antibiotics, but the only way to know is to test.
The drugs are structurally similar to each other and have the potential to cause a cross-reaction.
These common misconceptions can affect care and treatment options.
Gilbert Laurie Collection / Photographer’s Choice RF / Getty Images
What Is a Penicillin Allergy?
Penicillin allergies occur when the immune system misfires, mistaking the medication for a harmful invader.
This can lead to a potentially fatal reaction known as anaphylaxis.
Signs of a penicillin allergy typically begin within two hours of taking the drug.
These mild symptoms can be treated with an antihistamine like Zyrtec (cetirizine.
If you do not have an EpiPen, call 9-1-1 or go to the nearest emergency room.
A reaction after taking penicillin is not always an IgE-mediated response to the drug, though.
The symptoms are often a reaction to something else.
Side effects are also sometimes confused with a true allergy.
Historically, these side effects have been documented in medical records as “allergies.”
Today, severe allergic reactions to penicillins and other beta-lactam antibiotics are rare.
What Are Beta-Lactam Antibiotics?
Beta-lactamantibioticscontain a particular structure (called the beta-lactam ring) within their molecular makeup.
They fight bacterial infections by interrupting bacterial cell wall formation.
The beta-lactam class of antibiotics includes penicillin and penicillin derivatives, cephalosporins, and carbapenems.
Penicillin and Penicillin Derivaties
Penicillins and penicillin derivatives have generic names that end in -cillin.
These include:
Cephalosporins
Cephalosporins have generic names that begin with the prefix ceph- or cef-.
These include:
Carbapenems
Carbapenem antibiotics have generic names that end in -penem.
If you do not react to the oral challenge, the allergy can be removed from your medical file.
Anaphylaxis is less common and occurs in less than 0.1% of reactions.
There is no FDA-approved skin reagent to test for cephalosporin allergies.
If you have a history of suspected allergy, you should not try this at home on your own.
Research shows you’re free to react to cephalosporins and not penicillin.
It is also common to have an allergy to only one cephalosporin and no others.
However, more recent research found the cross-reactivity rate is much lower than previously thought.
Cross-reactions are more common with cephalexin, cefadroxil, cefprozil, and cefaclor.
Cefazolin, on the other hand, has a much lower cross-reactivity rate.
The risk of anaphylaxis to cephalosporins in persons who are allergic to penicillin is low.
The real-world implications were tested in a large California health system that uses automated alerts in electronic health records.
At the same time, rates of new allergies, severe allergic reactions, or anaphylaxis did not increase.
Can You ‘Outgrow’ a Penicillin Allergy?
Yes, you’re free to outgrow a penicillin allergy.
Summary
In the past, people allergic to penicillin were told to avoid cephalosporins.
However, more recent studies show a relatively small risk of cross-reaction.
True penicillin allergies are also less common than previously thought.
Additionally, some people are allergic to cephalosporins and not penicillins.
Research shows that most people outgrow a penicillin allergy within 10 years.
2021;156(4):e210021.
doi:10.1001/jamasurg.2021.0021
Center for Disease Control and Prevention.Evaluation and diagnosis of penicillin allergy for healthcare professionals.
Nemours Kids Health.Penicillin allergy.
American Academy of Allergy, Asthma, and Immunology.Penicillin allergy: what you should probably know.
American Academy of Allergy, Asthma, and Immunology.Anaphylaxis.
2016;6(8):a025247.
doi:10.1101/cshperspect.a025247
American Chemical Society.Discovery and development of penicillin.
2023;3(1):e153.
2019;34(50):e319.
2019;7(8):272238.e5.
doi:10.1016/j.jaip.2019.05.038
Zagursky RJ, Pichichero ME.Cross-reactivity in -lactam allergy.J Allergy Clin Immunol Pract.
2018;6(1):72-81.e1.
2021;4(4):e218367.
