Ondansetron is a 5-HT3 receptor antagonist antiemetic that works by blocking serotonin receptors in the chemoreceptor trigger zone. Ondansetron usually is part of an ongoing basal antiemetic regimen. Ondansetron also has been evaluated for use as a rescue medication to treat episodes of chemotherapy-induced nausea and vomiting (CINV) that are not otherwise controlled.