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Most people who think they are allergic to penicillin in fact are not, researchers said Friday. It’s something doctors have suspected for a long time, but the researchers say they were surprised by just how many people weren’t allergic to the antibiotic: it was 94 percent of them.

Dr. Thanai Pongdee, an allergist at the Mayo Clinic in Jacksonville, Florida and colleagues tested 384 people who said they were allergic to penicillin. Tests showed 94 percent of them were in fact, not allergic.

“These patients were scheduled to undergo orthopedic, general surgery, neurosurgery--any type of surgery you can think of,” Pongdee told NBC News. “We probably expected a little over half of people would not be allergic based on the time frame when they were initially determined to be allergic, but it ended up being a much higher proportion than that.”

"These patients are put on other antibiotics which be less effective and potentially have more side-effects.”

It’s good news for the patients, who can get cheaper, more generic penicillin-based drugs such as amoxicillin or related drugs called cephalosporins. These antibiotics generally cause fewer side-effects than other antibiotics, and they allow doctors to save the other drugs for penicillin-resistant infections.

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“There are two issues: these patients are put on other antibiotics which be less effective and potentially have more side-effects,” said Dr. James Sublett, a family allergist in Louisville, Kentucky who is incoming president of the American College of Allergy, Asthma and Immunology (ACAAI).

“The other is cost. A very commonly used substitute, for example Levaquin, is seven to eight times more expensive for a 10-day course than a similar course of generic Augmentin, a penicillin drug.”

People usually believe they are allergic based on a reaction in early childhood that they cannot even remember, says Sublett, but that they were told about.

“A patient will say, ‘My mom told me when I was 3 years old (that) I had a reaction to penicillin. And they’re going 30 years without being retested,” he said.

“We knew that the majority of people who list penicillin as an allergy actually aren’t allergic when they are reevaluated, so if you can determine they are not, you can avoid using more toxic and more expensive antibiotics,” Pongdee said.

It’s an important issue. The more widely an antibiotic is used, the more likely resistant “superbugs” are to develop – and that’s bad for everyone. While allergies are a serious matter, it’s a relief to know they are not as common as feared. “One of the first questions a patient is usually asked is ‘Are you allergic to any medications?’” Pongdee said.

“Some don’t realize there is really good allergy testing for penicillin."

The research, being presented at the annual meeting of the ACAAI, showed that it didn’t matter how severe a person’s first reaction to penicillin was. They usually outgrew it.

“It doesn’t happen very often that a health care provider challenges the presumption that the patient is still allergic. Many don’t realize that this is something a person may lose over time,” Pongdee said.

“Some don’t realize there is really good allergy testing for penicillin,” he added. “It’s a skin test. It takes about 30 minutes to do.”