A penicillin allergy is a common allergy. It occurs when your immune system reacts negatively to the antibiotic penicillin. You may experience a skin rash, swelling or difficulty breathing shortly after taking penicillin. An allergic reaction can be a medical emergency, so contact your healthcare provider or call 911 if symptoms are severe.
A penicillin allergy occurs when your immune system reacts negatively to the antibiotic penicillin or an antibiotic in the penicillin family (beta-lactam antibiotics).
Penicillin is a medicine that treats infections. When you take penicillin, the medicine destroys the outer wall of bacterial cells. This ultimately kills and removes a bacterial infection from your body.
Just because you have a penicillin allergy today doesn’t mean you’ll have that same allergy in the future. An estimated 80% of people diagnosed with a penicillin allergy lose their sensitivity to the medication after 10 years.
There are several types of penicillin that come as oral medicines that you take by mouth or as an injection. Types of penicillin include:
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A penicillin allergy can affect anyone whose immune system is sensitive to the medicine. You may have a higher risk of having a penicillin allergy if you have an underlying medical condition like hay fever, asthma or eczema.
A penicillin allergy is the most common medication allergy. About 10% of all people in the United States report having a penicillin allergy, but only 1% of those people actually have an allergy. A side effect, believed to be an allergic reaction, is the difference between the reported allergy number and the actual amount of people who have an allergy.
The most common symptoms of a penicillin allergy include:
You’ll most likely experience these symptoms within an hour after taking penicillin. In rare cases among adults, these symptoms can show up days to weeks after you’ve taken penicillin. Healthcare providers may see a reaction up to 7 days after prescribing a penicillin medication (like amoxicillin) to children.
The most dangerous reaction to penicillin is anaphylaxis, which is a severe, life-threatening allergic reaction. If you have any of the following symptoms, call 911, your local emergency services or visit the emergency room (ER):
If you believe you’re having a reaction to penicillin or to any medication you’re taking, call the healthcare provider who prescribed it to you. Call 911 or visit the emergency room if you have symptoms of anaphylaxis like swelling throughout your body or difficulty breathing. You may need emergency treatment to stop these symptoms.
There’s a difference between a side effect (adverse effect) of penicillin and an allergy. An allergy is usually more severe than a side effect. Many side effects are similar to those that you’d have during an allergic reaction, including a skin rash and itchy skin. Side effects are usually mild and typically go away after a few days. An allergic reaction can affect multiple parts of your body. A healthcare provider can offer testing to help you determine if you have a penicillin allergy.
You may have a penicillin allergy if you experience the following symptoms:
An allergic reaction can be life-threatening if left untreated. If you have severe symptoms, call 911 or your local emergency services number or visit the emergency room immediately.
Common side effects of penicillin include:
If you’re unsure whether you’re having an allergic reaction or a side effect to the medicine, contact a healthcare provider.
An immune system sensitivity to penicillin causes a penicillin allergy. Your immune system’s responsibility is to keep your body healthy by defending it against foreign substances like bacteria, fungi and viruses. During an allergic reaction to penicillin, your immune system mistakes penicillin as a negative foreign substance. This causes you to develop symptoms of an allergic reaction.
Some penicillin allergies appear the first time a person takes the medication. Symptoms may appear shortly after you take your first dose. For other people, the response appears the second time, after their immune system has had time to produce antibodies to it. An antibody is a protein made by white blood cells that helps defend against foreign substances like bacteria.
No, a penicillin allergy isn’t genetic. You can’t inherit the allergy if someone in your biological family has the same allergy.
A healthcare provider will diagnose a penicillin allergy after a complete medical history, physical exam and testing. Your provider will review your symptoms and they’ll offer immediate treatment if you’re having severe symptoms that could be life-threatening.
To confirm a diagnosis, your provider will perform an allergy test. During an allergy scratch test, your provider will drop two components of penicillin on your skin. A scratch is made through each drop. After 15 minutes, your provider will look for a skin reaction that includes skin discoloration, itchy skin and hives.
If the scratch test is negative, your provider may perform another type of allergy test called an intradermal test. During this test, your provider will use a small needle to place a tiny amount of penicillin underneath your skin. You’ll wait 15 minutes to see if you have a skin reaction, which will be a raised red bump (similar to a mosquito bite) at the site of the skin poke.
If both tests are negative, it indicates that you’re at low risk for a severe, immediate allergic or anaphylactic reaction to penicillin.
You may need to take penicillin under your provider’s supervision so they can monitor how you react. You’ll take a dose of penicillin or another penicillin-type drug like amoxicillin and you’ll be observed for 30 minutes to see how your body reacts.
Treatment for a penicillin allergy should be managed by your healthcare provider and could include:
Yes, there are alternative antibiotics available for you to take if you have a penicillin allergy. Notify your provider that you have an allergy to penicillin if they prescribe any medicine to you. For most people, having an allergic reaction to penicillin doesn’t mean that they’ll have an allergic reaction to other antibiotics.
If the condition for which you started taking penicillin hasn’t cleared up, your provider will most likely prescribe a different antibiotic that could include:
In the rare case in which there’s no other antibiotic that’s a safe and effective alternative, a provider may recommend drug desensitization therapy. In this therapy, you take a small amount of the drug, then gradually take more until you can take the recommended dose without having side effects. This may be done in a hospital so that expert care is available if there’s a serious problem. The effects of desensitization therapy may not be permanent. Some people may go through it again later in life.
You should feel immediate relief from some of your symptoms after you begin treatment. It could take a few weeks for a skin rash or hives on your skin to clear up completely. Your healthcare provider will let you know specifically what you should do to take care of yourself after having an allergic reaction.
It’s difficult to know if you’re allergic to penicillin if you’ve never been tested for it. Getting an allergy test is the best way to know what you’re allergic to so you can prevent a reaction. After you get an allergy test, you can notify each of your healthcare providers about your allergy so they don’t prescribe a penicillin-type antibiotic to you.
You should recover completely after a reaction from a penicillin allergy if you received quick treatment from a healthcare provider or the emergency department. Prompt treatment reduces your risk of having life-threatening symptoms.
If you have a penicillin allergy, you should tell all of your providers, including your dentist and any specialists you see. Bring it up before undergoing any type of treatment or procedure. Describe your reaction to penicillin so the people caring for you are fully aware of your risk factors.
Ask your healthcare provider if your penicillin allergy is severe enough for you to wear a medical alert bracelet warning others about it. If so, they can advise you on how to get one.
Many people who have a penicillin allergy will be able to take it again later in life. With skin testing and, in some cases, desensitization therapy, most people with a history of a penicillin allergy can safely take the medicine again. This is possible because your sensitivity to the drug can decrease with time. This isn’t the case for every person, so talk with your healthcare provider about your allergies. They can give you more guidance on this topic, as well as what other medications they recommend when you need antibiotics in the future.
If you don’t know you have a penicillin allergy, you may be at risk of developing an allergic reaction when you take penicillin. Ask your provider about allergy testing if you need to take penicillin.
If you have symptoms of a penicillin allergy after taking penicillin, contact your healthcare provider. If you have severe symptoms like widespread swelling throughout your body or difficulty breathing, call 911, your local emergency services number or visit the emergency room immediately.
A note from Cleveland Clinic
A penicillin allergy can be severe and life-threatening, so contact 911, your local emergency services or your healthcare provider immediately if you have a reaction after taking the medicine. You shouldn’t be afraid to take penicillin, because your body may adjust to it in the future. Your healthcare provider can help you find alternative types of antibiotics if you need to take them when you have a penicillin allergy.
Last reviewed by a Cleveland Clinic medical professional on 03/22/2023.
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Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. Policy