What is Zofran? Zofran (ondansetron) blocks the actions of chemicals in the body that can trigger nausea and vomiting. Zofran is used to prevent nausea and vomiting that may be caused by surgery or by medicine to treat cancer (chemotherapy or radiation). Zofran is not for preventing nausea or vomiting that is caused by factors other than cancer treatment or surgery. You should not use Zofran if you are allergic to ondansetron or to similar medicines such as dolasetron (Anzemet), granisetron (Kytril), or palonosetron (Aloxi).