Zofran^® is a prescription medication that is used to prevent nausea and vomiting associated with chemotherapy, radiation treatment, and surgery. The dose of Zofran will depend on several factors, including your age, the medical condition you are being treated for, and other medications you may currently be taking. As with any medication, there are some potential side effects of Zofran, such as headaches, fatigue, and constipation. Zofran works by blocking serotonin, a chemical produced by the body that is associated with nausea and vomiting. Zofran is designed to only prevent nausea and vomiting -- not to treat them once they begin.