( CNN ) -- A popular antibiotic used to treat bacterial infections can cause abnormal -- and possibly fatal -- heart rhythms in some patients , according to a new warning from the Food and Drug Administration . The drug , azithromycin , sells under the name Zithromax or Zmax and is commonly sold in what 's called a Z-Pak . The FDA warning comes after the agency reviewed a study on the drug that was published last May in the New England Journal of Medicine , as well as another study by a manufacturer of the antibiotic . The warning is `` not the result of adverse event reports related to azithromycin , '' according to an FDA spokeswoman . Researchers at Vanderbilt University looked at the records of thousands of Tennessee Medicaid patients over a period of 14 years . They found a 2.5 times higher risk of death from heart disease in the first five days of using a Z-Pak when compared to another common antibiotic or no antibiotics at all . The drug may cause changes in the electrical activity of the heart , according to the FDA . People with underlying heart problems seem to be especially vulnerable to developing this condition , said Wayne Ray , professor of preventive medicine at Vanderbilt and the study 's lead researcher . The FDA said patients who have low blood levels of potassium or magnesium are at a higher risk . Patients who have a slower than normal heart rate or are already taking drugs to treat arrhythmias should also be cautious . Ray told CNN it 's unclear why azithromycin can cause heart issues . He pointed out that similar antibiotics , such as amoxicillin , have also been found to cause heart arrhythmia . Pfizer , which produces the drug , released a statement Tuesday , saying : `` The majority of patients treated with Zithromax ( azithromycin ) are not affected by this label update . '' So should people with heart problems stop taking the drug ? Not necessarily . `` This just adds another 'con , ' '' said Ray . He said it 's important to ask your doctor the follow questions : How serious is the infection ? If I use a Z-Pak could it aggravate any underlying health issues ? Are there any other antibiotics that could work just as well ? Ray said doctors often prescribe Z-Paks because the drug only needs to be taken for five days , versus the 10-day periods that are typical with other antibiotics . Drug labels for azithromycin have been updated to include warnings about this risk , the FDA said .
Antibiotic sells under the name Zithromax or Zmax . Study found 2.5 times higher risk of death in the first five days of using the drug . Pfizer says majority of patients treated with the drug are not affected .