Quoted from http://www.boston.com/dailydose/2013/03/12/fda-warns-about-zithromax-and-risk-fatal-heart-problems/gwH9zWy3ZBBmMfRPpgx1wI/story.html
FDA warns about Zithromax and risk of fatal heart problems
The US Food and Drug Administration issued a warning today about the popular and powerful antibiotic azithromycin (Zithromax or Zmax), saying that it could lead to a potentially fatal irregular heart rhythm in patients who are at higher risk for heart problems. These include people with low blood levels of potassium or magnesium or a slower than normal heart rate, or who use certain drugs to treat abnormal heart rhythms, or arrhythmias.Patients who have existing QT interval prolongation -- a disorder of the heart’s electrical system -- or who have the rarer and more extreme form called torsades de pointes need to be especially cautious about using these antibiotics.