Penicillin Allergy

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What is a penicillin allergy?

A penicillin allergy is an allergic reaction that occurs when your body's immune system overreacts to these antibiotics. Many people who believe that they have an allergy to penicillin don't have it. They may have a side effect, rather than an allergic reaction. Tests can show if you have a penicillin allergy.

What puts you at risk?

Severe allergic reactions to penicillin can be deadly. You may be more likely to have this type of reaction if you have had:

  • A positive skin test for a penicillin allergy.
  • Hives that appeared quickly after you took the penicillin.
  • A bad reaction to penicillin in the past.

You are not likely to have an anaphylactic reaction to penicillin if you have had a rash with red, blotchy spots that appeared from a few hours to days after you took penicillin.

What are the symptoms?

A mild reaction can cause:

  • A rash.
  • A few hives.
  • An itchy nose or mouth.

A severe reaction to penicillin can cause an anaphylactic reaction. This can be deadly. Symptoms may include:

  • Raised, red areas (hives) all over your body.
  • Swelling of the throat, mouth, lips, or tongue.
  • Trouble breathing.
  • Passing out (losing consciousness).
  • Belly pain, nausea, vomiting, or diarrhea.

A severe reaction can also include a severe rash. The rash may have blisters and peeling.

The symptoms of a severe allergic reaction most often happen very soon after you take the medicine. But allergic reactions can happen later. If the reaction is later, you might have a rash, a fever, or other symptoms.

How is it diagnosed?

To find out if you have a penicillin allergy, your doctor will ask about your symptoms, such as:

  • What they were and how bad they were.
  • How soon they happened after you took the medicine.
  • How long ago they happened.
  • How they were treated.

You may get a skin test to check your reaction to penicillin. Your doctor may give you small doses by mouth. An allergic reaction most often happens quickly. You'll be watched closely while you have the test.

If the test is negative, then you are not allergic to the drug. You may never have been allergic. You may have had side effects instead of an allergic reaction. Or you may have lost the allergy over time.

How is a penicillin allergy treated?

The first thing to do to treat a penicillin allergy is stop taking the medicine. A mild reaction often can be treated with over-the-counter antihistamines. These medicines stop swelling and itching. Some people may need prescription medicine. For a severe reaction, you may need a shot of epinephrine or other medicines.

How can you care for yourself at home?

Avoid penicillin and medicines like it. Be sure that anyone treating you for any health problem knows what medicines you are allergic to. Take medicines for the allergic reaction as directed. If you are at risk for a severe allergic reaction, always have an epinephrine shot with you. Make sure it has not expired.

Related Information


Current as of: September 25, 2023

Author: Healthwise Staff
Clinical Review Board
All Healthwise education is reviewed by a team that includes physicians, nurses, advanced practitioners, registered dieticians, and other healthcare professionals.

The Health Encyclopedia contains general health information. Not all treatments or services described are covered benefits for Kaiser Permanente members or offered as services by Kaiser Permanente. For a list of covered benefits, please refer to your Evidence of Coverage or Summary Plan Description. For recommended treatments, please consult with your health care provider.