welcome back, everybody. an important medical item to pass along. the fda issued a warning abut a very popular antibiotic. a lot of folks have taken it, i have, too, the z-pak. >> concern that people that have heart conditions could suffer sudden death. abc's chief medical editor dr. besser tells us more about the medicine and who and when they should be concerned. >> reporter: prescribed 60 million times a year to treat common infections. ear, nose, throat, chest, so popular people ask for it by name. do you ever have a patient come in with a cold or flu who say doc, give me a z-pak. >> all the time. >> reporter: the warning from the fda for some patients, the z-pak or azithromycin can cause changes in the electrical activity of the heart which can be fatal. it applies to people with arrhythmias and people with low blood levels, potassium and magnesium.