The government, for the first time, is urging doctors not to prescribe two antiviral drugs commonly used to fight influenza because of concerns about drug resistance, officials announced Saturday.

The Centers for Disease Control and Prevention said the recommendation covers the drugs rimantadine and amantadine during the current flu season.

"Laboratory testing by CDC on the predominant strain of influenza (H3N2) currently circulating in the United States shows that it is resistant to these drugs," according to a CDC statement.

The two drugs have been used for years to combat the type-A, or most common, strain of influenza.

The CDC tested 120 influenza A virus isolates from the H3N2 strain and found that 91 percent, or 109, were resistant to the two drugs, according to the statement.

"This represents a sharp increase from last year when only 11 percent of isolates tested were resistant and 1.9 percent were resistant the year before that," the statement said.

One flu expert, Dr. William Schaffner of Vanderbilt University, said the development was "disconcerting" as flu now has joined the ranks of other diseases, such as tuberculosis and HIV, that recently have acquired the ability to resist front-line medications.

But Schaffner said doctors have other options to fight influenza.

One is the antiviral Tamiflu, also known as oseltamivir. The CDC said that all H3 and H1 influenza viruses the agency has tested so far are susceptible to the other commonly used antivirals, including Tamiflu and zanamivir.

"Tamiflu is now readily available everywhere — in most places, it is the primary antiviral being used" against flu, Schaffner said. "But we're always a bit frustrated when one of the therapeutic agents is foreclosed. It makes every infectious disease doctor worry a little bit."

Doctors also recommend an annual flu shot to help prevent getting influenza in the first place.

The CDC said it planned to alert doctors throughout the country via its emergency Health Alert Network and through a special edition of its weekly journal, the Morbidity and Mortality Weekly Report.

Each year, the flu kills about 36,000 people, and some 200,000 are hospitalized because of it in the United States, the CDC said. As of Dec. 31, the latest CDC data available, flu activity was only considered widespread in seven U.S. states, mainly in the Southwest and West: Texas, New Mexico, Arizona, Colorado, Utah, Nevada and California.