The letter is available at www.fda.gov/medwatch/safety/2008/safety08.htm#Relenzawww.fda.gov/medwatch
Information about neuropsychiatric events associated with zanamivir in people being treated for influenza was added to the warnings and precautions section of the antiviral drug's label.
The addition reflects postmarketing reports of delirium and abnormal behavior in patients with influenza taking neuraminidase inhibitors, including zanamivir (Relenza, GlaxoSmithKline). Some of the cases resulted in fatalities related to self-injurious behavior.
The addition is described in a Dear Healthcare Professional letter from GSK, and in a notice on the Food and Drug Administration's MedWatch site.
Most of the reports have been from Japan, primarily among pediatric patients, and events “often had an abrupt onset and rapid resolution,” the revised labeling says. The label says that the contribution of zanamivir to these events “has not been established” and points out that influenza also can be associated with various neurologic and behavioral symptoms, including seizures, hallucinations, delirium, and abnormal behavior, which in some cases can be fatal.
The incidence of these adverse events cannot be estimated because the reports were voluntary, “but they appear to be uncommon based on usage data for Relenza,” the label says.
Neuropsychiatric events associated with zanamivir and the other available neuraminidase inhibitor, oseltamivir (Tamiflu), were discussed at a meeting of the FDA's Pediatric Advisory Committee, and the panel narrowly recommended revising the labeling for both drugs to include information about these events.