In some people, azithromycin may cause an abnormal heart rhythm called QT prolongation. The chance of the condition is increased if you already have certain problems with your heart rhythm or if you take other drugs that could also cause QT prolongation. QT prolongation is very serious, and it could even be fatal in some cases. When you have any problems with your heart rhythm, tell your physician before taking azithromycin. Also tell your doctor about all other medications you’re taking prior to starting this drug. As the risk of heart health problems is higher with azithromycin than with various other antibiotics, people with heart disease or arrhythmias should ask their doctor about trying some other antibiotic.