WASHINGTON - The Food and Drug Administration is warning doctors and patients that a widely used antibiotic from Pfizer can cause rare but deadly heart rhythms in some patients.

The agency is adding new warnings to the label of Zithromax, which is commonly used to treat bronchitis, pneumonia and other infections.

Doctors should consider prescribing other antibiotics to patients at risk of heart problems, including those with irregular heartbeats or low levels of potassium or magnesium in their blood.

Zithromax is more expensive than other antibiotics, but it's popular because it often can be taken for fewer days.

The FDA warning could convince doctors to choose other options for people prone to heart problems, though the new label notes that some other antibiotics have similar effects on the heart.