"Antiviral medicines such as Tamiflu must be used with care and only when the medical situation justifies it," said Bj鰎n Olsen, of the Uppsala University and the University of Kalmar. "Otherwise there is a risk that they will be ineffective when most needed, such as during the next influenza pandemic."

While Tamiflu use is limited in most countries, some nations use the drug very heavily. "Use of Tamiflu is low in most countries, but there are some exceptions such as Japan, where a third of all influenza patients are treated with Tamiflu," explains co-researcher Jerker Fick.

Perhaps more worrying is the potential knock-on effect involving creatures that frequent water bodies. Influenza viruses are common among waterfowl, especially dabbling ducks such as mallards. These ducks often forage for food in water near sewage outlets. Here they could encounter Tamiflu in concentrations high enough to develop resistance in the viruses they carry.

"The biggest threat is that resistance will become common among low pathogenic influenza viruses carried by wild ducks," explains Olsen. "These viruses could then recombinate with viruses that make humans sick to create new viruses that are resistant to the antiviral drugs currently available." The researchers advise that the problem must be taken seriously so that humanity's future health will not be endangered by too frequent and unnecessary prescription of the drug today.

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Source: Public Library of Science