You should not use ondansetron if you are allergic to it or to similar medicines such as dolasetron (Anzemet), granisetron (Kytril), or palonosetron (Aloxi).
Before taking ondansetron, tell your doctor if you have liver disease, or a personal or family history of Long QT syndrome.
Ondansetron orally disintegrating tablets may contain phenylalanine. Tell your doctor if you have phenylketonuria (PKU).
Serious side effects of ondansetron include blurred vision or temporary vision loss (lasting from only a few minutes to several hours), slow heart rate, trouble breathing, anxiety, agitation, shivering, feeling like you might pass out, and urinating less than usual or not at all. Stop taking this medicine and call your doctor at once if you have any of these side effects. Ondansetron may impair your thinking or reactions. Be careful if you drive or do anything that requires you to be alert.