Now, the FDA is asking GlaxoSmithKline to go back and look at the safety profile for ondansetron...due to QT-prolonging effects: http://reut.rs/pDq6Yw They are already changing the labels to reflect cardiovascular risk in the meantime.
It should be interesting to see the results. It is fairly clear that ondansetron prolongs the QT interval probably nearly, but not quite, as much as droperidol. The droperidol black box was based on cardiovascular events including only a mere 10 patients receiving doses in the therapeutic range of 0.625mg to 1.25mg, and those events had multiple confounding factors or drug co-administrations. It would not surprise me if ample, if equally flimsy, evidence exists implicating ondansetron as well.
"Food and drug administration black box warning on the perioperative use of droperidol: a review of the cases."
"Droperidol and ondansetron-induced QT interval prolongation: a clinical drug interaction study."
"The effects of droperidol and ondansetron on dispersion of myocardial repolarization in children."