Penicillin allergy

Penicillin allergy is an overreaction by your immune system to penicillin and related antibiotics. If you have a penicillin allergy, your reaction to taking the antibiotic may range from a rash to anaphylaxis, a potentially life-threatening condition.

Garvan J. Lynch
MBA (Public Health)

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What is it?

Penicillin allergy is an overreaction by your immune system to penicillin and related antibiotics. If you have a penicillin allergy, your reaction to taking the antibiotic may range from a rash to anaphylaxis, a potentially life-threatening condition.

Penicillin allergy is the most common drug allergy. Penicillin antibiotics are widely prescribed for bacterial infections such as strep throat. However, not all unfavorable reactions to penicillin are a true penicillin allergy.

It isn't clear why some people develop penicillin allergy. Once you've had an allergic reaction to penicillin, the simplest way to prevent penicillin allergy is to avoid penicillin and related antibiotics.


Many people who report having a penicillin allergy don't have a true allergy. They may have had a reaction to penicillin, such as certain types of rash, but not all reactions are allergies. Penicillin allergy symptoms include:

Anaphylactic reactions

The most serious allergic reaction to penicillin is an anaphylactic response, which can be life-threatening. Anaphylactic reactions develop immediately after penicillin exposure in highly sensitive people. Signs and symptoms include:


Penicillin allergy occurs when your immune system responds to the drug as if it were a harmful substance instead of a helpful remedy. Your immune system triggers certain cells to produce immunoglobulin E (IgE) antibodies to fight the component of penicillin to which you're allergic (allergen). Chemicals released by your immune cells can cause the signs and symptoms associated with an allergic reaction.

Penicillin belongs to a family of drugs called beta-lactam antibiotics. These drugs include penicillin and amoxicillin, which are relatively inexpensive and effective at treating many common bacterial infections. Such infections include skin, ear, sinus and upper respiratory infections.

Taken orally or injected, penicillin works by stopping the growth of bacteria in your body. Several varieties of penicillin exist, and each targets a different infection in a different part of your body. You may have heard of some of the other drugs in the penicillin family, including:

  • Amoxicillin
  • Ampicillin
  • Dicloxacillin
  • Penicillin V

If you're allergic to one type of penicillin, you're at risk of being allergic to all penicillin-related antibiotics. Some people allergic to penicillin may also be allergic to cephalosporins, a class of antibiotics closely related to penicillin.

You aren't born allergic to penicillin, but you can develop penicillin allergy once you've been exposed to the drug. After that, re-exposure to penicillin or related antibiotics can trigger an allergic reaction, sometimes more severe than the reaction to the first exposure.

Risk factors

It isn't clear why some people develop penicillin allergies while others don't. However, certain people seem to be at greater risk of developing a penicillin allergy than others are.

You may be at higher risk if:

  • You're between the ages of 20 and 49
  • You've taken penicillin frequently
  • You have HIV/AIDS
  • You have cystic fibrosis
  • You've had allergic reactions to penicillin or another drug in the past


Your doctor will want to know about your past reactions to penicillin and may examine you to identify or exclude other medical problems. Unless you require penicillin — either because it's the only effective antibiotic for a life-threatening condition or because you're allergic to other antibiotics and have few treatment options — your doctor may prescribe another antibiotic.

Your doctor may recommend you be tested for penicillin allergy if:

  • You were allergic to penicillin in the past, but possibly are no longer sensitive to the drug
  • You require penicillin for treatment
  • Your doctor wants to keep you from taking a stronger antibiotic than your condition requires

Because of the risk of anaphylaxis, testing for penicillin allergy should only be done by an allergist in a hospital or doctor's office.

Skin test

This test, which can determine your sensitivity to the drug, involves the following:

  • A small amount of penicillin is injected into the skin of your forearm or back.
  • If you're allergic to the particular substance being tested, you develop a red, raised bump or reaction.

If the skin tests to penicillin are negative, your doctor may recommend penicillin or a related antibiotic. If the skin test to penicillin is positive, most likely your doctor will recommend that you continue to avoid penicillin and related antibiotics. 


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