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THURSDAY, April 8 (HealthDayNews) -- People who have had one allergic reaction to penicillin may not always have a second reaction to the drug, experts now contend.

New research finds that the number of reactions the second time around isn't as high as thought.

Still, the findings shouldn't be interpreted as carte blanche to prescribe the antibiotic to those who have had a previous reaction, says the study's author, Dr. Andrea Apter, an allergy-immunology specialist at the University of Pennsylvania. Her research appears in the April issue of the Journal of Allergy & Clinical Immunology.

"The overall rate is lower than quoted in the literature," she said. "Doctors say that 60 percent of those with an initial reaction will have problems the next time. In actuality, it rounds to about two percent."

Those numbers are based on the evaluation of more than three million electronic medical records of patients in Great Britain who received a prescription for penicillin from 1987 to September 2001, Apter said. Of that total, more than 2 million patients received at least two prescriptions for penicillin 60 days apart, and almost all of them had no adverse reaction to either dose.

There were, however, 3,014 patients among those who got two doses who had an adverse reaction to the first dose. But only 57 patients had an adverse reaction to the second course. Those 57, Apter said, represent 1.89 percent of the total number of patients.

"Having a problem the second time is much less common," she said.

Exactly why doctors give patients penicillin again when they've had problems remains unclear, Apter said. But sometimes people don't really know if they have had a penicillin allergy, she said.

Sometimes penicillin reactions are fairly immediate, she said. "If within an hour or two [of taking penicillin], they get rash, hives, shortness of breath, and there are no other reasons [for the symptoms], they know [they have an allergy]."

But sometimes the first allergic reaction occurred when people were very young and their memories of it aren't clear. Or a rash might appear days after the penicillin regimen is started, so the link isn't clear.

Penicillin skin testing is currently imperfect and remains commercially unavailable, Apter said.

The study shouldn't be interpreted to mean that those with a penicillin problem after the initial course can take it again without worry, Apter reiterated. "I wouldn't say someone who has had a problem with penicillin could safely have it again," she said.

More research is needed, she added.

Dr. Eric Macy, chairman of the Adverse Reactions to Drug and Biologicals Committee for the American Academy of Allergy Asthma and Immunology, said the new study points to the need to get accurate penicillin skin testing on the market as soon as possible.

"The materials to perform accurate penicillin skin testing are not commercially available anywhere in the world," Macy added. "There has been an orphan drug application on file with the FDA since 1987 for the penicillin skin test materials, but no pharmaceutical company has produced the materials for commercial sale."

"The AAAAI has started an initiative to bring pharmaceutical manufacturers, the Food and Drug Administration, the Centers for Disease Control and Prevention and large health-care providers together to get these important skin test materials produced and made available in the U.S.A. and the rest of the world," he said.

More information

The American Academy of Allergy Asthma and Immunology has more information on allergies to penicillin and skin testing for allergies.

(SOURCES: Andrea Apter, M.D., associate professor of medicine, and allergist and immunologist, University of Pennsylvania School of Medicine, Philadelphia; Eric Macy, M.D., allergist at Kaiser Permanente, San Diego, and chairman, Adverse Reactions to Drug and Biologicals Committee, American Academy of Allergy Asthma and Immunology; April 2004 Journal of Allergy & Clinical Immunology )

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