Introduction: Ondansetron is a 5-HT3 receptor antagonist commonly used as an antiemetic to prevent nausea and vomiting associated with anti-cancer drugs, cancer radiotherapy or postoperatively. Recently, the US Food and Drug Administration (FDA) issued a warning for ondansetron due to a potential for prolongation of the QT interval of the electrocardiogram (ECG), a phenomenon that is associated with an increased risk of the potentially fatal arrhythmia torsade de pointes. Areas covered: The authors undertook a review of the cardiac safety of ondansetron. Their primary sources of information were PubMed (with downloading of full articles) and the Internet. Expert opinion: The dose of ondansetron that the FDA has concerns about is 32 mg i.v. (or several doses that are equivalent to this), which is only used in preventing nausea and vomiting associated with cancer chemotherapy. This suggests that ondansetron may be safe in lower doses used to prevent nausea and vomiting in radiation treatment or postoperatively. However, as there is a report that a lower dose of ondansetron prolonged the QT interval in healthy volunteers, this needs to be clarified by the FDA. More research needs to be undertaken on the relationship between QT prolongation and torsades in order that the FDA can produce clear-cut evidence of proarrhythmic risk when introducing warnings for this.