Zithromax, also known as azithromycin or Z-Pak, substantially increases a patient’s chances for developing deadly heart problems, according to a 14-year Vanderbilt University study.

Zithromax, the commonly used Pfizer Inc. antibiotic, is prescribed to such ailments as bacterial respiratory infections, skin infections, ear infections, and sexually transmitted diseases. Patients who take the drug were found to suffer Zithromax complications that more than double their risk of death due to heart problems.

The Vanderbilt study analyzed health records and data on millions of prescriptions for several antibiotics given to some 540,000 Tennessee Medicaid patients – all of whom started out with comparable risks for heart trouble – between 1992 and 2006.

There were 29 heart-related deaths by patients taking Zithromax during five days of treatment, more than double that of patients on another antibiotic, amoxicillin, or those who took none. Zithromax patients with pre-existing heart problems were at highest risk.

“The results suggest there would be 47 extra heart-related deaths per 1 million courses of treatment with Zithromax, compared with amoxicillin,” according to CBS news. “A usual treatment course for Zithromax is about five days, versus about 10 days for amoxicillin and other antibiotics. Zithromax is at least twice as expensive as generic amoxicillin; online prescription drug sellers charge a few dollars per pill for Zithromax.”

In March 2013, the U.S. Food and Drug Administration (FDA) issued an announcement that azithromycin can cause abnormal changes in the electrical activity of the heart that may lead to a potentially fatal irregular heart rhythm.

“Patients at particular risk for developing this condition include those with known risk factors such as existing QT interval prolongation, low blood levels of potassium or magnesium, a slower than normal heart rate, or use of certain drugs used to treat abnormal heart rhythms, or arrhythmias,” according to the FDA.

The agency reported that drug labels had been updated to strengthen the Warnings and Precautions section with information related to the risk of “a specific, rare heart rhythm abnormality” known as QT interval prolongation and torsades de pointes.

The warning came, according to the FDA, after a review of a study by medical researchers in addition to another study by a manufacturer of the drug that looked at whether azithromycin could cause abnormal changes in the electrical activity of the heart.

Doctors should avoid prescribing azithromycin to patients prone to heart problems, according to officials. Though more expensive than other antibiotics, azithromycin is popular, in part, because it works more quickly than other antibiotics.