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