Ondansetron, also known as Zofran, is an anti-emetic drug - that is, a drug used to prevent vomiting and nausea. It, and related drugs like granisetron, is often used to combat the side effects of nausea-inducing drugs such as bleomycin, cisplatin and the antifungal amphotericin B, but may also be used to prevent nausea associated with certain illnesses or surgical anesthesia. Ondansetron itself has few side effects most of the time, but may occasionally cause headaches or constipation.

It is available in either an oral form (pills) or as an intravenous injection. The oral form is more often used to treat chronic nausea, while the IV form is used as pre-medication prior to anesthesia, infusions of anticancer drugs or amphotericin B. This sort of pre-medication prevents or reduces the otherwise rather traumatic vomiting attacks that these drugs can induce.

While ondansetron is considered by some to be a wonder drug, its extreme expense (upward of 40USD/dose) makes it less likely to be prescribed than some older antinauseants like Phenergan, Ativan and Reglan, which have negative side effects.