A drug used to prevent nausea and vomiting caused by chemotherapy and radiation therapy. It is also used to prevent nausea and vomiting after surgery. Ondansetron hydrochloride blocks the action of the chemical serotonin, which binds to certain nerves and may trigger nausea and vomiting. Blocking serotonin may help lessen nausea and vomiting. It is a type of serotonin receptor antagonist and a type of antiemetic. Also called Zofran.