Oseltamivir phosphate, which is available in oral formulation, is effective for type A/B influenza virus infection. In Japan, drug-induced adverse reactions such as central nervous system symptoms are a concern, especially in children. In this survey, we implemented a questionnaire regarding the administration period described on prescriptions and the duration for which patients took the drug, to investigate compliance with oseltamivir phosphate therapy. The results showed that therapy with oseltamivir phosphate was discontinued by 21.1%. The main reasons for discontinuation were the relief of symptoms and appearance of adverse reactions. This survey revealed that some patients taking oseltamivir phosphate would like to be prescribed medication for as short a term as possible. Although it is recommended that oseltamivir phosphate be administered for 5 days to treat influenza symptoms, 3- or 4-day administration was frequently prescribed for young patients (aged <20 years) and children. Because the appropriate administration period for influenza treatment is unclear, further epidemiological data are needed to determine the most rational use of neuraminidase inhibitors for influenza treatment.