A common antibiotic boosts the risk of cardiac arrest, especially when used with drugs often prescribed for infections and high-blood pressure, a U.S. study finds.

Erythromycin, which has been prescribed for five decades for everything from strep throat to syphilis, doubled the very low risk of the heart suddenly stopping among patients in the study.

That risk became more than five times as high – six deaths for every 10,000 people – among patients who took the antibiotic with certain other drugs for two weeks, the study finds.

The study shows the need to continue examining the safety of older medications and their interaction with newer drugs, said researcher Wayne A. Ray, a professor of preventive medicine at Vanderbilt University School of Medicine in Nashville.

The study – published in Thursday's New England Journal of Medicine – focused on commonly used erythromycin pills and medicines for infections and high-blood pressure.

Those included blood-pressure medications verapamil or diltiazem, which are sold as generics and under various brand names, including Verelan and Isoptin for verapamil and Cardizem and Tiazac for diltiazem.

Other drugs that posed a risk with erythromycin included:

• clarithromycin - an antibiotic sold under the Biaxin brand.
• fluconazole, or Diflucan - for vaginal yeast infections.
• ketoconazole (Nizoral) and itraconazole (Sporanox) - antifungal drugs.

Ray's team studied the medications used by 1,476 Medicaid patients from Tennessee who died of cardiac arrest from 1988 to 1993.

The study received funding from the U.S. Food and Drug Administration and two other federal health agencies, as well as the drug company Janssen Pharmaceutica, which makes Nizoral and Sporanox.