The U.S. Food and Drug Administration has warned that the popular antibiotic azithromycin, sold under the name Zithromax or Zmax or in the form of a Z-Pak, can result in irregular and possibly fatal heart rhythms in some patients.

Zithromax is prescribed for the treatment of a number of infections, including pneumonia, bronchitis, wheezing and COPD. The FDA states that doctors should take note of the risk posed by the drug and consider prescribing other antibiotics. Those with preexisting heart problems or low blood levels of potassium or magnesium are particularly at risk.

The warning comes after a study published last May in the New England Journal of Medicine along with another done by a manufacturer of the antibiotic. The study was conducted by researchers at Vanderbilt University, who examined records of thousands of Tennessee Medicaid patients over the course of 14 years. They found that those who used a Z-Pak had a 2.5 times higher risk of dying from heart disease in the first five days of using the drug than those who used a different antibiotic or no antibiotic.

The FDA is issuing a new warning to the azithromycin label that includes information regarding the increased risk of QT interval prolongation and a rare heart rhythm problem called Torsades de Pointes.

While it is unknown why azithromycin has the potential to cause heart problems, similar antibiotics, such as amoxicillin, have been known to cause heart arrhythmia.

In a statement released on Tuesday March 12, Pfizer, the producer of the drug, stated that, "The majority of patients treated with Zithromax (azithromycin) are not affected by this label update."

Professor of preventive medicine at Vanderbilt Wayne Ray, who led the study, stresses the importance of speaking with your doctor regarding the drug. He says that the new warning does not necessarily mean that those who use the drug should stop taking it.

"This just adds another 'con,'" he says.