For the second time in less than a year, the Food and Drug Administration (FDA) has revised the label of the antiemetic drug ondansetron (Zofran) in response to concerns that the drug may lengthen the QT interval, increasing the risk of torsades de pointes, a particularly lethal form of ventricular tachycardia. In September 2011, the FDA warned of this possible adverse effect and directed the manufacturer, GlaxoSmithKline, to complete a clinical trial specifically designed to determine the risk of QT-interval lengthening. Based on the company's preliminary findings, GlaxoSmithKline has announced that it's removing from the drug's label the option of using a single 32-mg IV dose as treatment for chemotherapy-induced nausea. It appears that this large single dose predisposes patients to torsades de pointes and, according to the June 29 FDA Drug Safety Communication (http://1.usa.gov/MF6mO9), "should be avoided." The FDA and the drug manufacturer are still evaluating data to determine the best alternative single dose.

Ondansetron achieves its therapeutic effect by selectively blocking serotonin 5-HT₃ receptors. It's still considered safe and effective to use in treating chemotherapy-induced nausea and vomiting in adults and children. The previously listed IV dosage recommendation, 0.15 mg/kg every four hours for up to three doses (with no more than 16 mg in a single IV dose), remains on the label. Recommendations on IV doses for the prevention of postoperative nausea and vomiting and on oral doses remain unchanged.

Nurses who administer IV ondansetron should assess patients for electrolyte abnormalities, such as hypokalemia or hypomagnesemia, which increase the risk of electrocardiographic (ECG) changes. Electrolyte abnormalities should be corrected before ondansetron therapy begins. Patients receiving ondansetron intravenously should be assessed for ECG changes, especially QT-interval prolongation. Clinicians should assess patients for risk factors for QT-interval lengthening (congenital long QT syndrome, congestive heart failure, bradyarrythmias, or concomitant use of medication that prolongs the QT interval). Patients at highest risk for QT-interval prolongation should be assessed most closely for ECG changes.