Oseltamivir (Tamiflu) is the most important antiviral drug available and a 
cornerstone in the defence against a future influenza pandemic. Recent 
publications have shown that the active metabolite, oseltamivir carboxylate 
(OC), is not degraded in sewage treatment plants and is also persistent in 
aquatic environments. This implies that OC will be present in aquatic 
environments in areas where oseltamivir is prescribed to patients for 
therapeutic use. The country where oseltamivir is used most is Japan, where it 
is used to treat seasonal flu. We measured the levels of OC in water samples 
from the Yodo River system in the Kyoto and Osaka prefectures, Japan, taken 
before and during the flu-season 2007/8. No OC was detected before the 
flu-season but 2-58 ng L(-1) was detected in the samples taken during the flu 
season. This study shows, for the first time, that low levels of oseltamivir can 
be found in the aquatic environment. Therefore the natural reservoir of 
influenza virus, dabbling ducks, is exposed to oseltamivir, which could promote 
the evolution of viral resistance.