Everything You Need to Know About Penicillin Allergy

By Craig Sorkin, DNP, APN
Medically reviewed checkmarkMedically reviewed
August 26, 2022

The antibiotic drug penicillin can be life-saving, but for the 1% of the population who has a penicillin allergy, it can be life-threatening. The thing is, nearly 10% of U.S. patients report penicillin allergies.

While it’s always important to avoid allergens when possible, and alternate medications work just as well, there’s no reason to avoid penicillin if you don’t have to. This article will help you determine if you may have a penicillin allergy.

First I’ll explain what penicillin is. Then I’ll discuss the symptoms, causes, and risk factors of a penicillin allergy. After that, I’ll share common side effects of penicillin, how to prevent an allergic reaction, and when to see a doctor about a possible penicillin allergy so you can receive a proper diagnosis. 

What Is Penicillin?

Penicillin is a common, affordable antibiotic drug used around the world.

It is effective against many types of bacterial infections, including: 

However, some types of bacteria have become penicillin resistant, which means that penicillin alone cannot kill them. When this happens, medical providers may prescribe other types of antibiotics or penicillin paired with another antibiotic.

Need antibiotics?

Our physicians can prescribe antibiotics for various conditions, but only if necessary. Chat with a provider to see which treatment option is best for you.

Get Started
Bottle image

Symptoms of Penicillin Allergy

Not all reactions to penicillin are allergic reactions.

If you have a penicillin allergy response, symptoms can include:

  • Sudden reactions that happen within one hour of taking
  • Raised pink or red hives that feel intensely itchy
  • Swelling of the face, extremities, stomach, genitals, or throat
  • Wheezing
  • Shortness of breath

Risk of anaphylaxis

In rare cases, penicillin allergic reactions can cause anaphylaxis. This is a severe reaction driven by an intense immune response.

It can include all of the symptoms of penicillin allergy listed above, as well as:

Delayed Reactions From Penicillin Allergy

A person can be allergic to penicillin but not have a reaction until days or weeks after exposure to the medication. This type of reaction may be caused by a different type of immune cell reaction, not one that is associated with a true immunoglobulin E (IgE) allergic response.

Though extremely rare, delayed adverse reactions from a penicillin allergy may include: 

  • Drug-induced anemia (reduced red blood cells)
  • Stevens-Johnson syndrome (severe blistering and skin peeling)
  • Nephritis (kidney inflammation)

Causes of Penicillin Allergy

A penicillin allergy happens when your immune system thinks penicillin is harmful and mounts an attack against it by producing antibodies. The first time you are exposed, you do not have a reaction. But when your immune system encounters penicillin again, it identifies it as a threat and produces an attack.

Penicillins are a common class of antibiotic drugs known as beta-lactam antibiotics. They combat bacterial infections by destroying the cell walls of the bacteria, which prevents them from continuing to replicate.

People who are allergic to one type of penicillin may or may not be allergic to other types of penicillin or beta-lactam antibiotics like cephalosporin.

Penicillin medications include:

  • Amoxicillin
  • Ampicillin
  • Dicloxacillin
  • Penicillin G
  • Penicillin V
  • Nafcillin
  • Piperacillin
  • Oxacillin
  • Ticarcillin

Drugs that are similar to penicillin are in the cephalosporin family.

These include:

  • Cephalexin (Keflex)
  • Cefaclor
  • Cefepime (Maxipime)
  • Cefadroxil
  • Cefotetan
  • Cefazolin
  • Cefprozil
  • Cefdinir
  • Cefuroxime

Cross-reactivity between cephalosporins and penicillin allergy is rare, about 10%, so someone who has a penicillin allergy can often take cephalosporins.

Risk Factors of Penicillin Allergy

It is not always possible to predict who may develop a penicillin allergy.

However, the following factors may increase the risk:

  • Having other allergies
  • Prior history of allergic reactions to drugs
  • Frequent penicillin use
  • Having a vagina
  • Having an illness associated with drug reactions (such as HIV or Epstein-Barr virus)

Penicillin Side Effects That Are Not Allergic Reactions

Penicillin may cause side effects that are not signs of an allergic reaction.

These side effects include: 

Preventing a Penicillin Allergic Reaction

If you have a true penicillin allergy, tell any medical providers and pharmacists to ensure that you are not given any type of drug that could provoke an allergic reaction. 

In addition to listing penicillin as an allergy in your medical records, consider wearing a medical alert bracelet to identify the allergy in case of an emergency.

Need antibiotics?

Our physicians can prescribe antibiotics for various conditions, but only if necessary. Chat with a provider to see which treatment option is best for you.

Get Started
Bottle image

When to See a Doctor

If you are taking penicillin or a drug in the same class and you experience side effects that you feel could indicate an allergic reaction, contact your medical provider. 

If at any point you show signs of anaphylaxis—such as difficulty breathing, chest pain, or an itchy rash—seek emergency medical care.

How a penicillin allergy is diagnosed

The following tests can determine if you have a penicillin allergy: 

  • A skin test is where a small amount of penicillin is injected into the surface of your skin. The provider then observes you for an hour to determine if any reaction develops. If one does, a penicillin allergy is confirmed. If there is no reaction, an oral challenge is the next step.
  • An oral challenge is where you are given a full dose of penicillin in a supervised healthcare setting. If you show signs of allergy, the medical provider will give you medication to stop the reaction. If you do not show signs of an allergic reaction after a certain amount of time, then you do not have an allergy.

How K Health Can Help

Did you know you can get affordable primary care with the K Health app?

Download K to check your symptoms, explore conditions and treatments, and if needed text with a healthcare provider in minutes. K Health’s AI-powered app is HIPAA compliant and based on 20 years of clinical data.

Frequently Asked Questions

What should you avoid if you are allergic to penicillin?
If you are allergic to penicillin, you may need to avoid all drugs in the penicillin class, which includes other popular medications like amoxicillin and ampicillin. You may also need to avoid cephalosporin medications like cephalexin (Keflex). Inform any healthcare provider and pharmacist of your allergy so you can minimize the risk of exposure.
How do I know I'm allergic to penicillin?
Penicillin allergies cause reactions within an hour of taking the medication. Symptoms include hives, itching, breathing problems, diarrhea, nausea, and vomiting. If you are unsure about a penicillin allergy, a medical provider can test you.
How long does an allergic reaction to penicillin last?
Allergic reactions to penicillin are similar to other types of reactions: Usually they come on quickly, within an hour of exposure. A severe reaction requires medical treatment to calm the immune system’s response. Less severe allergic reactions may last for up to a few days once you stop taking the medication.

K Health articles are all written and reviewed by MDs, PhDs, NPs, or PharmDs and are for informational purposes only. This information does not constitute and should not be relied on for professional medical advice. Always talk to your doctor about the risks and benefits of any treatment.

Craig Sorkin, DNP, APN

Craig Sorkin, DNP, APN is a board certified Family Nurse Practitioner with over 15 years experience. He received his Undergraduate and Graduate degrees from William Paterson University and his doctoral degree from Drexel University. He has spent his career working in the Emergency Room and Primary Care. The last 6 years of his career have been dedicated to the field of digital medicine. He has created departments geared towards this specialized practice as well as written blogs and a book about the topic.

K Health logo (used on certain page templates)