Oseltamivir (Tamiflu®) is the most important of the few antiviral drugs available for treatment of seasonal flu and a cornerstone in the defense against a future influenza pandemic. Most governments have built their preparedness plans around stockpiling oseltamivir and it is recommended by WHO both as treatment and prophylaxis in a pandemic situation.

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.
Yodo River

Yodo River in north Osaka

The researchers 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.
fetchObjectSampling locations in Kyoto (R1–R4) and Osaka (R5–R6) prefectures, Japan

Why is this a concern?

This study shows, for the first time, that low levels of oseltamivir can be found in the aquatic environment. Why is this a concern?  Influenza A virus is a zoonosis, with its natural reservoir in dabbling ducks.  Dabbling ducks, exposed to oseltamivir in these environments could promote the evolution of viral resistance resulting in the drug no longer being effective in the treatment of influenza.

PLoS ONE- Hanna Söderström, Josef D. Järhult, Björn Olsen, Richard H. Lindberg, Hiroaki Tanaka, Jerker Fick