Penicillin Allergies

Dr. Sharon Orrange
Dr. Orrange is an Associate Professor of Clinical Medicine in the Division of Geriatric, Hospitalist and General Internal Medicine at the Keck School of Medicine of USC.
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Many patients report a penicillin allergy based on family history or stories from their parents to stay away from penicillin without a true history of allergic reaction. Penicillin and its close relatives are amazing, cheap, and effective antibiotics so here are 10 things you need to know:

1. Serious allergies to penicillin are common, with about 10 percent of people reporting an allergy.

2. Most people who believe they are allergic, however, can take penicillin without a problem. This is because they were never truly allergic or because their allergy to penicillin has resolved over time.

3. Cool fact: only about 20 percent of people will be allergic to penicillin 10 years after their initial allergic reaction if they are not exposed to it again during this time period.

4. With a reported allergy, it’s not just penicillin you have to stay away from but the close relatives too: nafcillin, oxacillin, cloxacillin and dicloxacillin, ampicillin, amoxicillin, carbenicillin, ticarcillin, and piperacillin. This is a big deal, so we really need to know if you are allergic.

5. Rashes that involve hives (raised, intensely itchy spots that come and go over hours), or occur with other allergic symptoms like wheezing or swelling of the skin or throat, suggest a true allergy.

6. Several other symptoms can indicate that you are allergic to penicillin: angioedema (swelling around the face), wheezing, coughing, and trouble breathing from asthma-like reactions.

7. Anyone with a past history of this type of reaction may develop a more severe reaction, such as anaphylaxis, if you were to take the antibiotic again.

8. The big A. Anaphylaxis is a sudden, potentially life-threatening allergic reaction. Symptoms include those of an allergic reaction, as well as very low blood pressure, difficulty breathing, abdominal pain, swelling of the throat or tongue, and/or diarrhea or vomiting.

9. What can you do to find out if you really have a penicillin allergy? Skin testing for penicillin allergy is the most reliable way to determine if you are truly allergic to penicillin. A whopping ninety percent of people will test negative (meaning they do NOT have a penicillin allergy), because they either lost the allergy over time, or they were never allergic in the first place.

10. Skin testing should be done by an allergist in an office or hospital setting. Testing takes about an hour to complete. The skin is pricked and injected with weak solutions of penicillin and observed for a reaction. A positive test indicates that you are truly allergic. If you complete the skin testing without a positive reaction, a single dose of full strength penicillin is given to confirm that you don’t have an allergy to the medication. At that point, you are in the clear.

Dr O.

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