Study leader Jerker Fick of Umea University demonstrated that oseltamivir, the active substance in Tamiflu, passes virtually unchanged through sewage treatment.

In countries where Tamiflu is used frequently, there is a risk that its concentration in natural waters can reach levels where influenza viruses in nature will develop resistance to it, increasing the risk that influenza viruses infecting humans will become resistant to one of the few medicines currently available, the researchers said.

"Anti-viral medicines such as Tamiflu must be used with care and only when the medical situation justifies it," Bjorn Olsen of Uppsala University said in a statement. "Otherwise there is a risk that they will be ineffective when most needed, such as during the next influenza pandemic."

Oseltamivir is a substance is so difficult to break down means that it goes right through sewage treatment and out into surrounding waters, Fick said.

The study was published in the journal PLoS ONE.