Zithromax, a widely used antibiotic made by Pfizer, may cause a rare but deadly heart rhythm in some patients. According to Fox News, a study found that patients who took Zithromax had higher rates of fatal heart rhythms compared with patients who took other antibiotics. Pfizer is now adding new warnings to the Zithromax label, but the FDA’s warning could convince doctors to choose other options for people prone to heart problems.

More than 50 million people take the antibiotic Zithromax, which is also known as Z-Pak, every year. It’s commonly used to treat bronchitis, pneumonia, and other infections. Although several antibiotics have been known to interfere with the normal heartbeat, the FDA first issued a warning on Zithromax in May 2012 based on a Vanderbilt University study that was published in the New England Journal of Medicine.

Researchers at Vanderbilt found that Zithromax, which is also offered as a generic called azithromycin, were at an increased risk of cardiovascular death. Using health records and data from millions of prescriptions for several antibiotics given to about 540,000 Tennessee Medicaid patients from 1992 to 2006, researchers found the highest risks were in Zithromax patients with existing heart problems.

The FDA’s most recent warning, which was issued last week, is the result of a study conducted by the drug’s manufacturer. However, the manufacturer is not named and the study is not cited. According to the FDA, health care professionals should consider the risks of fatal heart rhythms with azithromycin when considering treatment options for patients who are already at risk for cardiovascular events.

People with low blood levels of potassium or magnesium, or a slower than normal heart rate, or people who use certain drugs to treat abnormal heart rhythms are considered part of the at-risk group. The FDA warns that the drug can alter the electrical activity of the heart, which may lead to a potentially fatal heart rhythm known as prolonged QT interval, in which the timing of the heart’s contractions becomes irregular.

Zithromax is more expensive than other antibiotics but it remains popular because it often can be taken for fewer days. In response to the FDA warning, Pfizer updated the Zithromax drug labels and stated that most patients will not be affected by this label change. The company also noted in an email that other antibiotics in the same class, known as macrolides, have similar risks, and Pfizer added, “Patients who are currently prescribed Zithromax should talk to their doctors of healthcare providers if they have questions regarding their treatment.”