Governments are stockpiling Tamiflu and Relenza. Stores in Mexico and in some parts of New York are running out of the two drugs as people seek swine-flu protection.

But the high demand for these antiviral drugs is a test of sorts, and no one knows how well they will perform if the outbreak becomes more severe.

There is no vaccine against this strain of swine flu, and developing one could take six months or more. Tamiflu and Relenza are the only bulwarks against the illness, scientists say.

An escalation of the outbreak will be a real-life experiment of how much protection the drugs provide should a flu pandemic erupt, said Brian Currie, medical director for research at Montefiore Medical Center in the Bronx and an infectious-disease specialist.

It's also a test of human behavior. After the 2001 anthrax scare, the antibiotic Cipro disappeared in six hours from Montefiore's pharmacy. Doctors prescribed it for each other and for relatives.

Already, some flu victims in New York said, their pharmacies have run out of Tamiflu.

So when the swine flu hit a few days ago, Currie immediately required high-level approval for Montefiore employees to fill a prescription for Tamiflu, which is more widely prescribed than Relenza.

Medical experts say people should not take Tamiflu or Relenza unless they have been exposed to someone with the flu or have been diagnosed with it. Nausea is a common side effect of Tamiflu, which is a pill. Relenza is inhaled and can cause respiratory problems in some patients, said Dan Hussar, a pharmacy professor at University of the Sciences in Philadelphia.

But fear may prevail.

"I would assume that there will be a problem with supplies of Tamiflu because a number of people will panic and start taking it without an appropriate indication," said Pascal James Imperato, a doctor and dean of the Graduate Program in Public Health at SUNY Downstate Medical Center in New York City.

At a Paris press conference yesterday, Severin Schwan, chief executive officer of Roche Holding AG, which makes Tamiflu, said the company was ready to boost production of the drug should the current outbreak become a pandemic. Last year, the Swiss drugmaker reported $531 million in Tamiflu sales.

GlaxoSmithKline, which is based in London and employs 4,500 in the Philadelphia area, said it also had "taken steps to increase production of Relenza," but a company spokesperson did not offer specifics. Relenza generated $105 million in 2008 sales. It is prescribed less frequently than Tamiflu because it is inhaled and thus more difficult for some patients, including children, to take.

Some strains of seasonal flu, which is different from swine flu, have been resistant to Tamiflu, which has boosted interest in Relenza.

Both drugs work by preventing the virus from leaving an infected cell to attack other cells, said Richard Wenzel, chairman of internal medicine at Virginia Commonwealth University Medical College and past president of the International Society for Infectious Diseases.

The drugs are believed to reduce the spread of the flu by about 70 percent, Wenzel said. That means that if 10 people are exposed to the flu, and then take one of the drugs, seven would not get the illness.

Their effect on those who come down with the flu is more modest, he added.

If given within 48 hours of infection, Tamiflu and Relenza both reduce the duration of the flu by about a day. That's worth it to many patients, Hussar said.

"Usually," he said, "people who experience influenza feel so miserable that shortening by any measure is welcome."

Contact staff writer Miriam Hill at 215-854-5520 or hillmb@phillynews.com.