It means stockpiles of the drug that governments started to build up five years ago in response to the outbreak of bird flu will not need to be thrown away before winter in the Northern Hemisphere, when the pandemic is expected to spread.
Some 220 million individual treatments of Tamiflu - each with 10 capsules to be taken over five days - have been sent to governments around the world since 2004 by the manufacturer, Swiss pharmaceuticals company Roche Holding AG, said spokeswoman Claudia Schmitt on Tuesday.
It is unclear how many of the treatments remain in stockpiles and how many have been used to treat outbreaks of bird flu and, more recently, swine flu, Schmitt said.
Antivirals such as Tamiflu can shorten swine flu illness by about a day if started within two days of the onset of the disease. GlaxoSmithKline's Relenza is believed to be similarly effective.
European Union members also have approved the extension of the shelf-life of Relenza, or zanamivir, from five years to seven years.
Tamiflu packages state that the drug should be used within five years of manufacture. Health authorities in Europe, the United States, Canada, Hong Kong and Australia have approved extending that to seven years for use against the pandemic strain of swine flu, said Schmitt.
The extension does not apply to Tamiflu's use against seasonal flu.
The decision may be in time to help some Asian countries that stockpiled the drug against the earlier outbreak of bird flu to avoid the expense of replacing it during the pandemic declared in June.
The World Health Organization accepts the decisions to approve the extension of the shelf life and it is acceptable to apply the extension retroactively in this case, said WHO spokesman Gregory Hartl.
"It is WHO's view that shelf-life extensions are a matter for national regulatory authorities," Hartl said.
WHO estimates that up to 2 billion people could become infected with the disease over the next two years, so it would be impossible at the current rate to provide doses of the drug to everyone infected. To help bridge the gap, Roche has licensed several other producers to make a generic version of the drug oseltamivir, which Roche markets as Tamiflu.
In addition, Schmitt said, "we are scaling up production of Tamiflu so we will be able if required beginning next year to produce about 400 million treatments per year."
She said Roche was still fulfilling the promise it announced in May to donate 5.65 million treatments of Tamiflu to WHO to help developing countries cope with the pandemic. That comes on top of an earlier donation of 5 million treatments.