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» Pandemics and scares  |  Flu 101  |  How vaccines work  |  Special Report

CDC: Two influenza drugs don't work

Doctors asked to stop prescribing amantadine and rimantadine

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ATLANTA, Georgia (CNN) -- With flu season reaching its peak, two drugs typically prescribed to fight the virus will be ineffective this season and should not be prescribed, the Centers for Disease Control and Prevention announced Saturday.

The CDC found in tests that the antiviral drugs amantadine and rimantadine were ineffective 91 percent of the time against H3N2 influenza, the dominant strain this season.

However, two other antiviral drugs, Tamiflu and Relenza are still effective, said CDC Director Dr. Julie Gerberding. Amantadine and rimantadine are older, less-expensive and less commonly prescribed drugs.

There has been a global trend toward drug resistance, but Gerberding said health officials didn't expect the resistance "to be quite as dramatic so soon." (Watch Dr. Marc Siegel explain how the strain became resistant to some drugs -- 2:44)

"We don't know exactly why resistance has risen to these drugs," Gerberding said, adding that the flu virus may have mutated, but there is no evidence to support that. "Flu constantly evolves, and we are always one mutation away from drug resistance."

There is no "magic bullet" to prevent drugs from becoming ineffective against certain flu strains, she said.

There is plenty of Tamiflu and Relenza for this flu season because the CDC stockpiled the drugs in case of an influenza pandemic, Gerberding said.

The flu is widespread in Arizona, Texas, New Mexico, Nevada, California, Oregon and Colorado.

The season, which can begin as early as October and stretch into May, typically is most widespread in December, January and February.

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