(WUSA 9) -- The Food and Drug Administration warns that the antibiotic, azithromycin, known as Zithromax or Z-pack, can cause life-threatening, abnormal heart rhythms.
The patients who are at greatest risk are those who already have an irregular heartbeat, a slow heart rate or low levels of potassium or magnesium.
Among patients with these specific factors, a study published in May in the New England Journal of Medicine found that the risk of cardiovascular death was 1 in 4,100 patients. The risk increases the longer the patient has taken zithromax.
The study found a higher risk of cardiovascular death in patients taking zithromax versus patients taking no antibiotics or taking the antibiotic, amoxicillin.
The FDA has already set preventive measures into action. According to MedPage Today, the "FDA is strengthening the warnings-and-precautions section of the drug's label to highlight the risk of QT prolongation and torsades de pointes."
QT prolongation and torsades de pointes are irregular heartbeat conditions that can lead to fainting or even sudden death.
More than 40 million Americans received a prescription for azithromycin in 2011. The antibiotic is used to treat many kinds of infections including pneumonia and tonsillitis.
A spokesperson from Pfizer sent this statement:
"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, in collaboration with the US FDA, has updated the Zithromax (azithromycin) label with regards to a specific, rare heart rhythm abnormality in patients with risk factors such as 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 ("arrhythmias").
It is important to note that other macrolide antibiotics are similarly labeled.
"It is also important to note that the majority of patients treated with Zithromax (azithromycin) are not affected by this label update. This potential risk is well established in macrolide antibiotics, as well as other antibiotics, such as quinolones.
"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.
Patients who are currently prescribed Zithromax should talk to their doctors or healthcare providers if they have questions regarding their treatment."