Everything You Need to Know About Penicillin Allergies

A penicillin allergy is one of the most commonly reported drug allergies. A lot of people are diagnosed with a penicillin allergy in childhood, but many of these allergies are very mild. Some people with mild allergies can actually safely take penicillin as an adult. 

It’s important to know whether you have a mild or more serious penicillin allergy. If you have a more serious allergy, you’ll need to avoid penicillin and similar antibiotics. But if you have a mild allergy, you don’t have to avoid these medications. That can make things easier for you if you get an infection. It also helps in the fight against antibiotic resistance.

Let’s take a look at penicillin allergies and what you can do to make sure you use the right antibiotics.

What is a penicillin allergy?

Our immune system is designed to protect us from harmful invaders. A drug allergy happens when our immune system views a medication as a foreign invader and triggers a response to fight against it. People with an allergy to penicillin have an inappropriate immune system response to penicillin. Unfortunately, that response can cause uncomfortable reactions or even dangerous symptoms, like anaphylaxis. 

Is it common to be allergic to penicillin?

Penicillin allergy is the most commonly reported drug allergy. While 10% of people think they have an allergy to penicillin, studies have shown that most of those people only have very mild penicillin reactions. So they’re not at risk for serious symptoms like anaphylaxis. In fact, only about 1% of the U.S. population has a serious penicillin allergy. 

So why is it that 90% of people think they have a penicillin allergy actually don’t? There are two main reasons.

1. Mislabeled allergy

In real time, it can be tough to tell the difference between an allergic reaction and symptoms of a viral illness. 

Picture this: A healthcare provider sees a child with an earache and fever. The eardrum looks red and swollen. The provider prescribes amoxicillin to treat an ear infection. But after 48 hours, the child develops a rash. Is it an allergic reaction to amoxicillin? Maybe. But a virus — not a bacteria — may have caused the ear infection. And that rash may simply be a symptom of the viral illness. This scenario is extremely common. As a result, some people are mislabelled as having a penicillin allergy when they don’t really have one.  

2. Resolved allergy

Penicillin allergies can go away. Most people with an allergy to penicillin will lose that allergy over time. Even if they had an allergic reaction to penicillin as a child, they might not be allergic to penicillin now. So although that allergy is gone, people continue to avoid penicillin antibiotics because they think they still have an allergy. 

What are the symptoms of a penicillin allergy?

Penicillin allergy symptoms usually appear within 1 hour of taking the antibiotic. The most common symptom is an allergic rash from penicillin. 

Some of the other symptoms of an allergic reaction to penicillin are:

• Hives, a raised, itchy skin rash

• Coughing or wheezing

• Tightness in the throat 

• Swelling of the face

• Feeling lightheaded or faint

• Fast heartbeat

• Confusion

• Nausea and vomiting

• Abdominal pain

The most serious reaction is anaphylaxis, which is an immediate, severe allergic reaction. Anaphylaxis can be deadly. If you take penicillin — or any antibiotic — and develop any of these symptoms, seek emergency care right away.

Does it matter if you have a penicillin allergy?

It’s important to know if you have a serious penicillin allergy. If you do, you need to avoid penicillin and other useful antibiotics. This can have unfortunate consequences. 

For example, researchers compared people labeled with a penicillin allergy to those without a penicillin allergy who were admitted to the hospital. The people with a penicillin allergy tended to have longer hospitalizations, received more antibiotics, and were more likely to develop a serious infection. 

Can I be tested for penicillin allergies?

The good news is there’s a simple penicillin allergy test — penicillin skin testing. It’s a good idea to get tested if you aren’t sure whether you have a penicillin allergy or if you had only a minor allergic reaction years ago.

A healthcare provider can do allergy skin testing in their office. Testing takes about 1 hour. During the test, your provider will place solutions of penicillin under your skin. Then they’ll check to see if you develop a reaction. 

The test is positive if your skin develops a raised, itchy area at the site of the injection. That means you’re allergic to penicillin. If you don’t have a reaction, your provider may ask you to do a “drug challenge test.” During this test your provider gives you a single dose of amoxicillin and then monitors you to confirm that you don’t have an allergy to the medication. If you take the medication and don’t have a reaction, you don’t have a penicillin allergy.

What antibiotics should I avoid if I have a penicillin allergy?

With a penicillin allergy, it’s not just penicillin you have to stay away from. You’ll also need to avoid its close relatives, including:

• Amoxicillin

• Amoxicillin clavulanic acid (Augmentin)

• Nafcillin

• Oxacillin

• Cloxacillin 

• Dicloxacillin

• Ampicillin

• Carbenicillin

• Ticarcillin

• Piperacillin 

Of these medications, the ones to be most aware of are amoxicillin and augmentin. If you’re allergic to penicillin, you can’t take amoxicillin or augmentin. Amoxicillin and augmentin can treat ear infections, skin infections, and lower respiratory tract infections. 

In the past, people with penicillin allergy were also told to avoid taking cephalosporins, a class of antibiotics. This group includes medications like cephalexin, cefazolin, and ceftriaxone. People with a penicillin allergy are at slightly higher risk of allergic reaction to cephalosporins, but previous studies may have overestimated that risk. Less than 5% of people with a penicillin allergy are also allergic to cephalosporins. 

If you have a penicillin allergy, talk to your healthcare provider before using these antibiotics. You may not need to avoid them altogether, but you may need observation for signs of allergic reactions.

What antibiotics can I take if I have a penicillin allergy?

If you’re allergic to penicillin, you may be wondering about which antibiotics you can take. Some oral medications you can instead of penicillin or amoxicillin include:

• Clindamycin

• Sulfamethoxazole/trimethoprim (Bactrim)

• Doxycycline

If you’re hospitalized with a serious infection, you may need treatment with vancomycin, an intravenous (IV) medication. Vancomycin is safe to take even if you have a penicillin allergy. 

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Written by:

Christine Giordano, MD

Christine Giordano, MD, is board-certified in general internal medicine. She received her medical degree from Rutgers New Jersey Medical School and completed residency at Thomas Jefferson University.

Edited by:

Patricia Pinto-Garcia, MD, MPH

Patricia Pinto-Garcia, MD, MPH, is a medical editor at GoodRx. She is a licensed, board-certified pediatrician with more than a decade of experience in academic medicine.

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