Oseltamivir is a neuraminidase inhibitor that inhibits influenza virus proliferation, and is used as an antiviral drug against influenza A and B viruses. Recently, concerns have been raised about hallucinations, delirium, and abnormal activity after administration of oseltamivir for treatment of infection with influenza virus. A large epidemiologic study was conducted in Japan in the winter of 2006-2007 to assess the relationship between oseltamivir intake and adverse behaviors, and an interim report was released on 10 July 2008. In the report, the research group concluded that no positive associations were detected between oseltamivir intake and abnormal behaviors. However, the analytic method used in the study was flawed. A correct analysis (based on person-time) shows a rate ratio of 1.57 (95% confidence interval = 1.34-1.83). This example clearly illustrates the importance of person-time in the analysis of cohort studies.