What is ondansetron and how does it prevent nausea?

Ondansetron, brand name Zofran, is an antiemetic that works by blocking serotonin 5-HT3 receptors in the chemoreceptor trigger zone and the vagus nerve, preventing nausea and vomiting signals from reaching the vomiting center in the brain. It is highly effective for chemotherapy-induced, postoperative, and radiation-induced nausea and vomiting. Ondansetron can be given orally, intravenously, or as an orally disintegrating tablet. Side effects are generally mild and include headache and constipation.