Penicillin allergies: What do I need to know?

Key Points

• Penicillin is a commonly-prescribed antibiotic that is used to treat a variety of bacterial infections.
• It is estimated that around 10% of U.S. patients report a penicillin allergy, but less than 1% have a true penicillin allergy.
• An allergic reaction to penicillin may occur within minutes, hours, or days of taking the medication.
• Symptoms of a penicillin allergy may include hives, swelling, light-headedness, and difficulty breathing.
• Skin testing and challenge doses can confirm a suspected penicillin allergy in certain situations.

How rare is it to be allergic to penicillin?

While 10% of U.S. patients report being allergic to penicillin, less than 1% are truly allergic upon further evaluation. Still, penicillins are one of the top causes for medication-related anaphylaxis. An allergy to penicillin can develop at any age. It can also go away over time. Around 90% of patients identified with a true allergy to penicillin are no longer allergic over a 10 year period.

What are the symptoms of a penicillin allergy?

Symptoms of a penicillin allergy can range from mild to severe. The most common symptom of a penicillin allergy is a skin rash. It can take 1-2 weeks for the rash to resolve after stopping the medication.

Common symptoms of a penicillin allergy include:

• Skin rash
• Itching
• Flushing
• Hives
• Swelling

Tell your healthcare provider if you experience any of these symptoms while taking penicillin.

Serious symptoms of a penicillin allergy include:

• Chest tightness
• Fast heartbeat
• Trouble breathing
• Wheezing
• Voice changes
• Vomiting
• Dizziness
• Fever
• Blistering skin
• Mouth sores

If you have any of the serious symptoms listed above, get medical help right away. You could be having anaphylaxis, which is life-threatening and needs to be treated immediately.

How quickly does a penicillin allergy show?

Allergic reactions can generally be split up into two categories: immediate reactions and delayed reactions. It is possible to experience either type of reaction from penicillin, and the timing is very different.

Immediate reaction

If you have an immediate reaction to penicillin, symptoms begin within 1 hour of taking a dose. In some cases, it can take a little longer – even up to 6 hours. Taking the medication with food could delay the time it takes for symptoms to occur.

Keep in mind that you may not experience any symptoms with the first dose, but could experience symptoms within an hour of a later dose. Even if you were able to take penicillin without any problems in the past, it is possible to have an immediate reaction to it during another course. This is due to a process called allergic sensitization.

Delayed reaction

If you have a delayed reaction to penicillin, it could take days after you start treatment for symptoms to show up. This means you might not experience symptoms of an allergic reaction to penicillin until a week or more after starting it. For this type of reaction, you might not have symptoms for many hours after a dose.

How do I know if I'm allergic to penicillin?

Your allergist or other healthcare provider can help you determine if you have a true penicillin allergy. They will take a careful medical history and ask you about your symptoms and the timing of your reactions. It can be hard to differentiate a penicillin allergy from symptoms of an infection or a nonallergic reaction, such as stomach upset. Two ways to confirm a penicillin allergy include skin testing and challenge testing.

Skin testing

During a skin test, a healthcare professional will prick your skin with two types of penicillin. They will also place a small amount of penicillin under your skin. If the test is positive, your skin will look red with a raised bump. A positive skin test means you are allergic to penicillin.

There should not be significant skin changes if the test is negative. A negative skin test means you likely do not have a true penicillin allergy. Your healthcare provider may recommend a challenge dose of penicillin in a healthcare setting to make sure.

Related questions

• Allegra vs Zyrtec: What's the difference?
• Amoxicillin rash: When should I be concerned?
• Can you take hydroxyzine for anxiety and sleep?

Challenge testing

A penicillin dose challenge can be helpful to determine the risk of an immediate allergic reaction. During challenge testing, a small amount of penicillin is taken by mouth. If you tolerate the small dose, a larger dose may be given after 30-60 minutes. This may be repeated until the full dose is tolerated. If a patient is able to tolerate the full dose of penicillin, they are considered to be not allergic to it.

Skin testing and challenge testing are not appropriate for patients who have had severe allergic reactions to penicillin in the past, including Stevens-Johnson syndrome, toxic epidermal necrolysis, and drug rash with eosinophilia and systemic symptoms (DRESS).

What is penicillin allergy treatment?

Treatment of a penicillin allergy will depend on your specific symptoms. For a rash, consider trying an oral antihistamine like diphenhydramine. This can help with itching, too. If the rash is severe, your healthcare provider might recommend topical or oral steroids.

More serious symptoms such as swelling in the face and trouble breathing require urgent medical treatment with epinephrine. This can be given through an autoinjector or at the nearest emergency room. Other treatments include albuterol to open up the airways, IV fluids to raise blood pressure, and steroids to help with inflammation.

What should you avoid if you are allergic to penicillin?

If you have a penicillin allergy, you should avoid medications belonging to the drug class known as penicillins. Penicillins are a group of beta-lactam antibiotics, and include the following medications:

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

There is also a small risk of having an allergic reaction to a closely-related group of antibiotics called cephalosporins. While earlier studies reported cephalosporins to have a 10% cross reactivity with penicillin, more recent studies have shown this is likely much lower. Many patients allergic to penicillin can tolerate a cephalosporin antibiotic, but if you have had a severe allergic reaction it is best to discuss the risks and benefits with your healthcare provider.

This is not a full list of medications that belong to the penicillin class. Always check with your healthcare provider before starting new medications, and let them know if you might be allergic to penicillins.

What is penicillin desensitization?

If you are allergic to penicillin but need to take a penicillin antibiotic, a healthcare professional can perform what is called penicillin desensitization. This is a process that involves giving penicillin gradually so it is tolerated without an allergic reaction. Penicillin desensitization is always done under the supervision of a healthcare provider, and is usually done in a clinic or hospital setting.

After desensitization, you are likely to have an allergic reaction once you have stopped taking the antibiotic for more than 24 hours and then take it again. If you need to take penicillin again in the future, you will need to have another desensitization procedure.

What antibiotics are safe if you are allergic to penicillin?

Alternative antibiotics that you can take with a penicillin allergy will depend on the type of infection you are treating. Many times, a broad-spectrum antibiotic is recommended instead of penicillin. Antibiotics from other classes include doxycycline, ciprofloxacin, clindamycin, and vancomycin. Read this article to learn more about antibiotics that are safe to take if you have a penicillin allergy.