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Do You Really Have a Penicillin Allergy?

Chances are, you don't, doctors say, and avoiding this antibiotic carries serious health risks

A pill bottle with a Penicillin label sitting in a medicine cabinet with a toothbrush and other toiletries.

Leonard Gertz, Getty Images

If you’re like the one in 10 Americans who think they have a penicillin allergy, you’ve probably been avoiding the drug for decades. Not only is that probably unnecessary, but it could be dangerous for your health, experts say.

Unlike unnecessarily avoiding peanuts, not taking penicillin when it would normally be prescribed can have serious health consequences for an individual as well as for society. A recent study of over 300,000 patients in the U.K. over six years is the first to show the extent of the danger. Those who reported a penicillin allergy — and thus were given alternative antibiotics — had 69 percent more staph infections (specifically, the antibiotic-resistant Staphylococcus aureus [MRSA]) and 26 percent more Clostridium difficile infections than those without a penicillin allergy. Those infections can be difficult to treat and even life-threatening. So it’s more important than ever, researchers say, for those who think they have a penicillin allergy to find out if they really do.

“We want to use the most narrowly effective antibiotic possible, and if allergies are getting in the way of that, gather more information” about the suspected allergy, says Kimberly Blumenthal, M.D., assistant professor of medicine at Harvard Medical School and lead author of the study, published this summer in The British Medical Journal. She says that less than 1 percent of people who have a penicillin allergy on their record get it verified. It’s possible that even a brief conversation with your doctor could clear up the confusion. Otherwise, a simple skin test is available to determine a true allergy.

Penicillin is very targeted. It can kill invasive bacteria while leaving others unharmed. When patients report an allergy to penicillin, they are usually prescribed an alternative antibiotic. Those alternatives are broad spectrum, meaning they kill the bacteria causing the infection but also destroy other beneficial bacteria in the body. That can wreak havoc on the body’s microbiome. “When you start to broad-brush and kill off good bacteria that normally keeps other bacteria in check, bacteria like C. difficile can thrive and cause serious problems, including diarrhea, dehydration, sepsis and death,” explains New Jersey anesthesiologist Anita Gupta, who adds that older people's guts tend to be more fragile and, therefore, more at risk of such problems.


Using broad-spectrum antibiotics also leads to more drug-resistant bacteria — superbugs, like MRSA — which leave everyone more vulnerable. “If we can stop the use of these broad-spectrum drugs that kill everything, we won’t see as much C. difficile and resistant bacteria,” says Michael Blaiss, M.D., executive medical director at the American College of Allergy, Asthma and Immunology.

So why is it that, as a recent study showed, 90 percent of patients who think they have a penicillin allergy can actually tolerate the drug? “In the ‘50s, ‘60s and ‘70s, [penicillin] was used like candy,” says Bruce Gordon, M.D., a Massachusetts otolaryngologist. “A lot of kids had viral infections and developed rashes while taking penicillin. But it was because of the virus, not the penicillin. Still, nobody wanted to risk it because of the few people who have true and severe allergic reactions.”

Allergists say that both patients and doctors have been frequently confused by what a true allergic reaction to penicillin looks like. It's not, for instance, vomiting. Actual reactions include itchy hives, a swelling of the face and other areas, wheezing or difficulty breathing, and they occur within a few hours of taking the drug. “The timing is important; this happens within a few hours, not like hives that appear two weeks later,” Blaiss explains. “A lot of doctors say, ‘You might be allergic to penicillin. Just avoid it.’ ”

Many of those who were told as a child they had a reaction have spent decades avoiding penicillin. Yet after 10 years, most people outgrow the allergy, especially if it's a more typical and less severe one, Gordon notes. 

Naturally, if you’ve thought you are allergic to the antibiotic and avoided it your whole life, you still might not want to take a chance on a penicillin prescription. One solution is to see an allergist for a simple skin test. (You can’t get the test while on some medications, including beta blockers or antihistamines; check with your doctor about suspending them for the test, Blaiss says.) 

While at the allergist's office, you're watched for hives or swelling after a skin prick. If nothing happens, you might be given an oral dose of penicillin and kept in the office for about an hour to be sure you don’t have a reaction. Most patients don’t. “I’ve seen one person in 35 years develop some wheezing and hives after the test,” Blaiss says. “It’s done in an office, and any allergist is equipped to handle an anaphylactic reaction.”

A discussion with your doctor might reveal that such a test isn't even necessary. “If a history says a week later they had vomiting, I’ll just say, ‘Next time you can take penicillin,’ ” Blaiss says.

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