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Are you really allergic to Penicillin?

Approximately 10% of people in the US report having a penicillin allergy to their doctor. Avoiding all penicillin type antibiotics can drastically reduce the dr...

Approximately 10% of people in the US report having a penicillin allergy to their doctor. Avoiding all penicillin type antibiotics can drastically reduce the drugs that your doctor can select and put you more at risk of running out of options if your infection is due to a resistant strain.

People tie up all types of adverse events as allergies but this is a mistake that can have important consequences. With dozens of drugs built off of the basic penicillin core called the beta-lactam ring, you don’t want to exclude their use if you don’t have to. Only around 1% of people have immunoglobulin G (or IgG) mediated allergies to penicillins, the ones that result in hives, throat, mouth, and facial swelling which can cause wheezing and can be life threatening. They should not take penicillins like ampicillin, amoxicillin, and Augmentin® since the risk of having a recurrence is high. Hives look like little blisters and are fluid filled. Diarrhea and upset stomach are two things that people consider penicillin allergies but they are not, they are just side effects of the medications. Similarly, a body rash that is made up of little red dots is called a macular rash, they are not hives. A macular rash isn’t pleasant but it is not life threatening so if there are no other good options, it is reasonable to use a penicillin again and just accept that a rash may result.

If you cannot remember, it may be worthwhile to have a skin test. Skin tests can safely determine if people do not have a true allergy 97% of the time. This is best to do before you are deathly sick due to an infection so when the time comes, your doctor will know what options are available.   The next thing we need to talk about are the cousins of the penicillins, the cephalosporins which include drugs like Keflex®, Ceclor®, and ceftriazone.

Before 2005, people assumed if you had a true penicillin allergy, you could not take a cephalosporin either. However, the reality is that in people with true penicillin allergy, only about 2% of people also have an IgG mediated allergy to a cephalosporin and in many cases, the cephalosporin has a common side chain with a penicillin. For example, amoxicillin and ampicillin share a side chain with Keflex®, Ceclor®, and four other cephalosporins but not with the other 19 cephalosporins.

HOST: So what is the take home message, what should people do with this information?

DR WHITE: If you know that you have something other than hives, trouble breathing, or the swelling we discussed, don’t tell people that you have an allergy. If you are not sure or if you had always avoided it because a family member had a penicillin allergy, get skin tested to determine whether or not you can be treated with this class of drugs. If you truly have a penicillin allergy, you may still find a cephalosporin that could work for you but your doctor may not know about the latest research and a simple reminder from you can help them evaluate your options with fresh eyes.

Table: Penicillin drugs that are more likely to cross-react with a cephalosporin

Penicillin Names Penicillin G Ampicillin

Amoxicillin (Augmentin®)

Caphalosporins That Might Cross-React Cephaloridine



Cefaclor (Ceclor®)



Cephalexin (Keflex®)


Dr. Michael White, UConn School of Pharmacy

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