Nov. 8, 2022 Danielle Penick has been allergic to penicillin since she was a baby – or so she thought. She had avoided the antibiotic for decades after having developed a nasty rash when her pediatrician prescribed the drug to treat an ear infection.

But when she came down with a sinus infection while pregnant in her early 30s, the supposed allergy became a problem. 

“The doctor said she had limited options because penicillin is one of the safest options for pregnant women,” Penick, now 38, recalls. 

When Penick wanted an allergy test the most, she couldn’t get one, she says. Doing so could have put her unborn baby at risk because the test would have exposed her to the drug to see how her immune system responded. A bad reaction – such as her airways closing and not being able to breathe – could be dangerous for her, and could result in a lack of oxygen for her unborn baby. 

Experts say many people who have a reaction to medication as kids never follow up with further testing. But that initial, bad reaction is often not an allergy at all. Sometimes the response is a symptom of the infection or a side effect of the treatment.  

Of those who are truly allergic to penicillin, about half will completely outgrow the allergy or lose sensitivity within 5 years. Even more will outgrow the allergy after 10 years. 

“Penicillin is the most commonly reported drug allergy,” says Zachary Rubin, MD, a pediatrician specializing in allergy and immunology at Oak Brook Allergists in Chicago. “But when you actually do studies to see if people are truly allergic, it’s estimated that 9 out of 10 people who say they are allergic aren’t really allergic.”

Penicillin is an important allergy to have re-evaluated for several reasons, Rubin says. 

The cheap, effective antibiotic is generally well-tolerated. Studies also show people with penicillin allergies spend more time in the hospital, and are at higher risk of becoming sick with bacteria that have developed resistance to antibiotics, making it harder for treatments to work. 

When people report a penicillin allergy, doctors prescribe other antibiotics. Studies show that those other drugs – which are good if you need them – can be more expensive, less effective, and more likely to cause adverse reactions than penicillin. 

Penicillin isn’t the only allergy people can outgrow. 

Food allergies are even more commonly outgrown than allergies to drugs – specifically those to egg and milk, says Payel Gupta, MD, an assistant clinical professor at State University of New York Downstate Medical Center and Mt. Sinai Medical Center, in New York City. 

How allergies work 

People can be allergic to not only medications but foods, insect venom, trees, cats, dogs, cosmetics, and more. 

An allergic reaction involves antibodies, which are produced by the immune system help fight infections and keep you from getting sickAntibodies react the same way to allergens – but allergens aren’t always harmful.

When someone is allergic, their body has an abnormal immune response to a foreign substance, like a peanut or penicillin. The immune system produces an antibody called immunoglobulin E, or IgE. 

“The body has these IgE antibodies that recognize the allergen and cause the body to release chemicals that cause the symptoms of allergies,” – congestion, itchy eyes, hives, wheezing, difficulty breathy, swelling of the eyes, lips, or tongue, Gupta says. 

The body essentially sees these allergens as a threat, and the immune system reacts accordingly, triggering symptoms. 

Some allergic reactions are more severe and require people to carry an epinephrine device, such as an EpiPen. Epinephrine, also known as adrenaline, works to relax airway muscles if someone has an anaphylactic reaction. 

Rubin says people outgrow allergies for many reasons, some of which scientists don’t yet understand.  

Often with food allergies, the body produces less IgE in response to the allergen over time, or dampen the immune response to the allergen. 

How and when to get evaluated

Rubin encourages people to think about the last time they had a reaction to the allergen in question. If it was a month ago, perhaps keep avoiding it. But if the problem occurred many months or years ago, a visit to the doctor might be in order. 

Gupta agrees, especially if the most recent allergic reaction was more than 10 years ago. 

“This is especially true for female patients who are thinking about getting pregnant and have a penicillin allergy,” she says. 

As for Penick, after she gave birth to her daughter, she made an appointment with her allergist to find out if she could safely take penicillin. 

She received an answer to her question within an hour: She wasn’t allergic to the drug.

Now, Penick says, she’s motivated to get tested for other allergens. 

She has one other suspected allergy to medication – lidocaine, a numbing agent often used in epidurals and dental work. She had a bad reaction once when she broke her arm in her 20s, but she was never sure if it was the anesthetic or something else. 

“It’s changed my thinking, knowing I can get some answers,” she says. 

Show Sources

Zachary Rubin, MD, pediatrician specializing in allergy and immunology, Oak Brook Allergists, Chicago.

Payel Gupta, MD, assistant clinical professor, State University of New York Downstate Medical Center, Mt. Sinai Medical Center in New York.

New England Journal of Medicine Journal Watch: “Penicillin Allergy Is Associated with Longer Hospital Stays and Increased Antibiotic Use.”

Cleveland Clinic, “Antibodies.”

© 2022 WebMD, LLC. All rights reserved. View privacy policy and trust info