The Centers for Disease control issued a warning to doctors that two commonly-used drugs prescribed to fight influenza should no longer be used, as the primary strain of the virus has built up resistance to the drugs.

According to the CDC, 91 percent of the samples test proved resistant to rimantadine and amantadine, up last year from only 11 percent.

CDC officials said that the newer antiviral drugs, Tamiflu and Relenza, were more effective. However, health officials worried about the supply of the two newer drugs in the face of fears about the spread of the bird flu.

Officials also worried that the virus would build up resistance to the newer drugs over time as well.

"Clinicians should not use rimantadine and amantadine ... because the drugs will not be effective," said CDC director Dr. Julie Gerberding.

Gerberding told reporters that the agency didn't know how the resistance occurred, saying it may have been the result of a mutation in the virus or overuse of the drugs abroad, such as in countries that permit the drugs to be purchased without a prescription.

The CDC said it would alert doctors all over the country through the emergency Health Alert Network and through a special edition of the Morbidity Mortality Weekly Report