Currently, medications that block the 5-HT3 receptor are widely accepted as a first-line anti-emetic in both the prevention and treatment of PONV. Ondansetron is the most commonly prescribed. An intra-venous dose of Ondansetron administered intra-operatively is predicted to reduce PONV by 15-20% when compared to patients who receive a placebo. However, Ondansetron has a half-life of approximately six hours and must be re-dosed in patients with prolonged or delayed-onset PONV.