An antibiotic used to treat common infections may carry serious heart risks, according to a new warning from the U.S. Food and Drug Administration.

The drug, called azithromycin but sold under the brand names Zithromax and Zmax as Z-Pak capsules, is prescribed for infections of the ears, lungs, sinuses, skin, throat, and reproductive organs, according to the FDA. But the antibiotic can interfere with the heart's electrical activity, disturbing its rhythm with potentially fatal consequences.

"Health care professionals should consider the risk of fatal heart rhythms with azithromycin when considering treatment options for patients who are already at risk for cardiovascular events," the FDA said in a statement.

Elderly people and those with irregular heart rates, arrhythmias, and low blood levels of potassium or magnesium are at a particularly high risk for the deadly heart condition, according to the FDA.

The warning comes 10 months after a study published in the New England Journal of Medicine found a small increase in cardiovascular deaths among people treated with Zithromax compared to those given the antibiotics amoxicillin or ciprofloxacin or no treatment at all. At the time, the FDA urged patients taking Zithromax to "not stop taking their medicine without talking to their healthcare professional" - a recommendation that stands today, despite the new warning.

Z-Pak antibiotics carry heart risks. (Image credit: azithromicin-rx.net)

Zithromax, made by Pfizer, comes in tablet and liquid form and is usually taken for up to five days. The drug's labels have been updated to strengthen the warnings and precautions section.

"Patient safety is of the utmost importance to Pfizer and we continuously monitor the safety and efficacy of our products to ensure that the benefits and risks are accurately described in the product label, as approved by the FDA," Pfizer said in an email statement to ABCNews.com. "Zithromax (azithromycin) has had a well established benefit risk profile for more than twenty years and continues to be an effective treatment option for patients all over the globe suffering from many types of bacterial infections."

Pfizer noted that the majority of patients treated with Zithromax "are not affected by this label update."

While alternatives to Zithromax are available, the FDA warned that they, too, can carry "other significant side effects that should be considered when choosing an antibacterial drug."

People taking Zithromax should seek immediate medical care in the event of an irregular heartbeat, shortness of breath, dizziness or fainting, the FDA said.