Health authorities in the U.S. are advising doctors to stop using two drugs to treat this winters influenza because it has become evident that the most common strain has become resistant to the drugs and they are unlikely to work.

The warning is regarding the antiviral drugs Amantadine and Rimantadine and only applies to the seasonal influenza; it does not apply to the the H5N1 bird flu strain.

Julie Gerberding, director of the Centers for Disease Control and Prevention, says tests of 120 samples of the H3N2 flu strain, the major cause of flu in the United States, has shown it was resistant to Amantadine and Rimantadine in 91 percent of cases.

Apparently during the last flu season, only 11 percent of cases were resistant to the two drugs.

Gerberding says doctors should not use Amantadine and Rimantadine to prevent flu or to treat patients suspected of having influenza because the drugs will not be effective, and should instead prescribe either of two newer medicines, Tamiflu or Relenza.

It seems that Amantadine and Rimantadine are generic medicines used less commonly than the newer drugs as they are cheaper; they are sometimes prescribed to nursing home patients during a flu outbreak to prevent infection.

Gerberding does not expect the new advice to affect a large number of patients.

She says it is unclear whether the flu virus spontaneously mutated or if something else caused the rise in resistance.

However many experts believe sales of the drugs overseas without a prescription, may have increased exposure enough for the virus to adapt and evade the medicines.

According to Gerberding flu cases are now widespread in seven U.S. states, mostly in the South and Southwest regions, but it is too early to tell if this year’s outbreak has peaked.

Countries have been stockpiling the drugs Tamiflu and Relenza to guard against the event that the bird flu virus mutates to a form that is easily transmitted amongst people.

Gerberding says there are ample supplies of both drugs to meet the needs for the regular flu this season.