The problem is that it prolongs the QT interval, and Azithromycin which they combine it with for it's great anti-inflammatory properties also prolongs the QT interval. Now what the hell is that? So, first you gotta know about EKG's, so here's a labelled one (source:
SinusRhythmLabels ), now let me break this down P = atrial depolarization (upper two chambers of your heart basically getting charged, and ready to contract), PR segment = atrial contraction (where they dump the blood into your ventricles), QRS complex = ventricular depolarization (bottom two chambers getting charged, ready to contract), ST segment = ventricular contraction (heart pumps the blood out into your body and through the lungs), T = ventricular repolarization (ventricles are in a "stunned" state, they can't contract again, and kind of reset). QT interval refers to the time it takes for everything to happen through the ventricle. Prolonging this period has to do with messing with potassium levels, and it ends up causing your heart not to beat in the proper response, this unsynchronized heart beat ends up leading to Torsades de Pointes, which looks like this:
Torsades, as you can see, not a good time to have that type of heart electrical activity. I hope that all made sense.