Influenza illness is an important cause of severe morbidity and mortality in the population. Oseltamivir, the first oral neuraminidase inhibitor, has proven efficacy. In children of 1 year and older (weight-dependent dosing: 30 mg, 45 mg, 60 mg or 75 mg BID for 5 days) and adults (75 mg BID for 5 days), oseltamivir reduces the duration and severity of acute influenza. Furthermore, it decreases the incidence of secondary complications such as otitis media, bronchitis, pneumonia and sinusitis. Oseltamivir has been shown to prevent influenza when given for long-term prophylaxis or for post-exposure prophylaxis. Because oseltamivir blocks the neuraminidase, an enzyme crucial to influenza virion liberation from the host cell, it is only effective during the replication phase. Clinical benefits are only seen, when oseltamivir is applied within 48 h after onset of symptoms, and clinical efficacy in acute influenza is highly dependent on the beginning of treatment. Treatment within 12 h after onset of symptoms reduces the duration of illness by an additional 74.6 h, and treatment within 24 hours an additional 53.9 h compared to the benefit seen with an intervention at 48 h. In conclusion, clinical efficacy of oseltamivir can be maximized by early start of treatment. Resistance of influenza virus against oseltamivir has rarely been observed and seems to be of no clinical relevance due to reduced transmissibility and pathogenicity of mutants. Oseltamivir is generally well tolerated. About 10% of the patients complain of transient upper gastrointestinal events, which resolved within 1-2 days, and which could be reduced when the medication was taken with a light snack.