Are Cephalosporins Safe If You're Allergic to Penicillin?

Cephalosporins
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A common question asked by people who have a history of a penicillin allergy is if they can take a cephalosporin. This is because penicillins and cephalosporins are antibiotics that are structurally similar to each other.

Before answering this question, though, it's important to first determine whether or not a person is truly allergic to penicillin—a common misconception that can affect your care.

Penicillin: A Beta-Lactam Antibiotic

Penicillin is perhaps the most well-known member of a group of antibiotics called beta-lactams. All of the beta-lactam antibiotics contain a particular structure (called the beta-lactam ring) within their molecular makeup.

Besides penicillin, other beta-lactams include:

  • Penicillin derivatives like amoxicillin
  • Cephalosporins like Keflex (cephalexin)
  • Carbapenems like Primaxin (imipenem)

Penicillin Allergy: An IgE-mediated Response

The most common allergic reaction to penicillin is an IgE-mediated (Type 1) hypersensitivity response. This means that when exposed to penicillin, a person's immune system (if allergic) will develop IgE antibodies. These antibodies travel and bind to certain cells within a person's body, causing them to release chemicals. These chemicals are what cause the symptoms of an allergic reaction.

Symptoms and signs of a penicillin allergy usually start within minutes to an hour or two of taking the drug and may include one or more of the following:

  • Skin Itching and/or hives
  • Itching or tingling of the lips or tongue
  • Angioedema (swelling of the mouth, eyes, lip, or tongue)
  • Wheezing and shortness of breath
  • Difficulty swallowing
  • Low blood pressure
  • Lightheadedness
  • Fast heart rate

Why Determining a True Penicillin Allergy Is Important

According to the Centers for Disease Control and Prevention, about 10 percent of all Americans report an allergy to a penicillin-class antibiotic, yet less than one percent are truly allergic (based on skin testing).

Perhaps, they were erroneously labeled as penicillin allergic in their chart or their allergy resolved with time— about 80 percent of people with a penicillin allergy lose their hypersensitivity after 10 years (meaning they are not allergic anymore).

Since many people who think they are allergic to penicillin are not, it's important for you and your doctor to tease this potential misconception out. This is because broad-spectrum antibiotics are often given as an alternative to penicillin.

Broad-spectrum antibiotics can increase your risk for antibiotic resistance in the future and are often more costly.  Also, the alternative antibiotic prescribed by your doctor may not actually be the best one for your infection.

Confirming a Penicillin Allergy: Skin Testing Is the Gold Standard

In order to determine whether or not you have a penicillin allergy, your doctor will take a drug allergy history and based on that information determine whether penicillin skin testing (which requires an allergy referral) is needed.

For example, perhaps you developed nausea or a yeast infection when last taking penicillin. This is not a true allergy, but rather a nonallergic side effect.

Or maybe you have never taken penicillin but you have a family member who has a penicillin allergy.

This, again, is not an allergy on your part. In these cases, your doctor will likely go ahead and treat your infection with a penicillin or a cephalosporin (if applicable).

On the flip side, if you are unsure about your prior reaction to penicillin, or if you doctor remains uncertain whether the reaction was allergic or not, an evaluation by an allergist should be done.

Safety of Taking a Cephalosporin With a Penicillin Allergy

Let's say you see an allergist, and your skin test is positive for a penicillin allergy. In this case, you should avoid all penicillins. 

However, you may still be able to take a cephalosporin under the close guidance of your allergist.

This is because only a small percentage of people (around 3 percent) with a penicillin allergy will react to a cephalosporin—however, the reaction can be severe. 

The type of cephalosporin you are taking will also be considered. This is because while research shows a low risk of reacting to a cephalosporin in people with a penicillin allergy, there is one exception—first-generation cephalosporins like cephalexin (Keflex) and cefazolin (Ancef) cause higher rates of allergic reactions in people with a penicillin allergy than people without a penicillin allergy. 

On the flip side, second- and third-generation cephalosporins, such as cefuroxime (Ceftin), cefprozil (Cefzil), cefdinir (Omnicef), and cefpodoxime (Vantin), do not appear to cause more allergic reactions in people with a penicillin allergy.

A Word From Verywell

The bottom line is that while most people with a history of a penicillin allergy can tolerate cephalosporins, many doctors use caution when prescribing one, as the reaction has the potential to be severe.

In addition, it's important to understand that there are many factors involved in the decision to prescribe a cephalosporin, like the type of infection that needs to be treated and the appropriateness/availability of non-penicillin/non-cephalosporin antibiotics that could be used.

Sources:

Blumenthal KG, Shenoy ES, Hurwitz S, Varughese CA, Hooper DC, Banerji A. Effect of a drug allergy educational program and antibiotic prescribing guideline on inpatient clinical providers' antibiotic prescribing knowledge. J Allergy Clin Immunol Pract. 2014 Jul-Aug;2(4):407-13.

Blumenthal KG, Solensky R, Allergy evaluation for immediate penicillin allergy: Skin test-based diagnostic strategies and cross-reactivity with other beta-lactam antibiotics. 

D ePestel DD et al. Cephalosporin use in treatment of patients with penicillin allergies. J Am Pharm Assoc (2003). 2008 Jul-Aug;48(4):530-40.

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