Oseltamivir and zanamivir are currently licensed worldwide for influenza treatment and chemoprophylaxis. Both drugs require twice-daily administration for 5 days for treatment. A new influenza drug, laninamivir (code name R-125489), and its prodrug form, CS-8958 (laninamivir octanoate or laninamivir prodrug), which are long-acting neuraminidase inhibitors, are introduced in this review. Laninamivir potently inhibited the neuraminidase activities of various influenza A and B viruses, including subtypes N1-N9, pandemic (2009) H1N1 virus, highly pathogenic avian influenza (HPAI) H5N1 viruses and oseltamivir-resistant viruses. Because of the long retention of laninamivir in mouse lungs after an intranasal administration of CS-8958, therapeutic administration of a single dose of CS-8958 showed superior efficacy to repeated administrations of zanamivir or oseltamivir in animal infection models for influenza A and B viruses. These include pandemic (2009) H1N1 virus and HPAI H5N1 virus. Prophylactic single administration of CS-8958, as early as 7 days prior to infection, also showed superior efficacy. Finally, the potential of a single inhalation of CS-8958 for influenza patients was demonstrated by clinical studies, and CS-8958 has been approved and is commercially available as Inavir(®) (Daiichi Sankyo Co., Ltd, Tokyo) in Japan.