The FDA is warning patients about a popular antibiotic used to treat bacterial infections.
The drug Azithromycin is commonly sold under the name Zithromax, in what's called a Z-Pak.
The agency said it can cause abnormal and possibly fatal heart rhythms in some patients. FDA officials said the warning was issued after they reviewed two studies on the drug.
A Vanderbilt University study found an increased risk of death from heart disease in the first five days of using a Z-Pak when compared to other common antibiotics or no antibiotics.
The FDA said patients who have low blood levels of potassium or magnesium are at a higher risk.
Patients who have a slower than normal heart rate or who are taking drugs to treat irregular heartbeats should also be cautious.
Pfizer, which produces the drug, said the majority of patients using the antibiotic are not affected.