By MURTAZA AKHTER

The U.S Food and Drug Administration is investigating the antibiotic azithromycin, commonly known as Z-Pak, after a study linked the drug to an increased risk of death.

The study, published Wednesday in the New England Journal of Medicine, found patients prescribed Z-Pak were more likely to die than those prescribed amoxicillin, another antibiotic. The results were especially pronounced for those who died of   heart attacks, strokes, sudden cardiac death and other cardiovascular causes.

Last year, doctors wrote 55.3 million prescriptions for Z-Pak, according to IMS Health. Like the popular antibiotic amoxicillin, azithromycin is commonly prescribed to help fight off bacterial infections. But while they are often prescribed for similar conditions, the two drugs work differently from each other.

Wayne Ray, a professor of preventive medicine at Vanderbilt University and lead author of the study, says he believes many doctors prescribe azithromycin instead of amoxicillin because of its easier regimen; patients on azithromycin take fewer pills over fewer days, which means they are more likely to finish their entire course of medicine. Finishing the entire course not only leads to more effective treatment, but it also lessens the risk of the patient developing a drug-resistant bug.

But, Ray says of azithromycin, "the risk of death may outweigh convenience and compliance, at least for high-risk patients."

These high-risk patients would seem to be those who have certain types of cardiovascular disease. Some doctors have suggested that azithromycin may cause irregular heartbeats known as arrhythmias, which in some people can lead to death. In Ray's study, which looked at Medicaid patients from 1992 to 2006 and analyzed millions of prescriptions, he and his colleagues found that patients on azithromycin had two and a half times the odds of dying from a cardiovascular cause while taking their medication than did patients on amoxicillin. The gap was widest when looking at the 10 percent of patients with the worst cardiovascular disease.

The study couldn't tease out which patients were on azithromycin because they had allergies to amoxicillin.

Cardiologist Leonard Ilkhanoff, director of the Inherited Arrhythmia Program at Northwestern University's Feinberg School of Medicine  in Chicago, says that while it's impossible to know what precisely is causing the increased risk of death, the study is very interesting. But he cautions that patients on azithromycin should not panic.

"Patients shouldn't be thinking, 'If I'm on azithromycin, I'm going to die,'" he says, adding that sometimes azithromycin really is the proper antibiotic for a patient. "It's appropriate, though, to discuss the risks and benefits with your doctor."

The FDA currently recommends that patients taking azithromycin continue to take the drug.