By Toni Clarke

BOSTON (Reuters) - Pharmacies across the country are struggling to meet demand for antiviral medications as anxious patients rush to fill prescriptions, raising the specter of supply shortages for those who really need treatment.

Prescriptions for Tamiflu and Relenza, which are believed to mitigate symptoms of the new Influenza A H1N1 flu, also known as swine flu, are skyrocketing.

Demand for the two drugs, as well as older antivirals that do not work against swine flu, are running nearly nine times higher than normal for this time of year, according to SDI, which provides healthcare analysis for the pharmaceutical industry.

In big metropolitan areas such as New York and Los Angeles, demand is running as much as 16 times higher than normal.

So far, hoarding has not prevented supplies reaching people who actually fall sick, and distributors such as Cardinal Health Inc and AmerisourceBergen Corp say they still have plenty of supply. In addition, the U.S. government has 50 million doses in a national stockpile and recently released 25 percent of those doses.

But if the new flu continues to spread or worsen, distributors say they may be instructed by the government to redirect supply to the most needy areas, and restrictions may be placed on physicians to ensure the drugs are prescribed only to people who need treatment.

"Right now there is no formal effort to restrict the use of Tamiflu, but if the spread continues there may be more prescription regulation," said Dr. Joseph Stubbs, president of the American College of Physicians. "Our guidelines will largely reflect what the CDC guides us to do."

Current guidelines from the U.S. Centers for Disease Control and Prevention, or CDC, are to write prescriptions for patients who have a confirmed or suspected case of swine flu or those who are exposed to an infected person and have an underlying condition such as heart disease.

Those are guidelines that physicians are not always able to follow when faced with panicky patients. So far the new flu has killed up to 176 people in Mexico, shutting down large parts of the country and raising fears of a global epidemic.

"It is very appropriate for us at CDC to create guidance and we think it is useful, but as a physician I know that the reasons to treat patients are based on a whole range of factors," said Dr. Joe Bresee, a pediatrician and chief of the epidemiology and prevention branch of the influenza division at CDC.

According to Bresee, the CDC's guidelines are likely to change soon as the traditional flu season ends and patients showing up in doctors' offices with fever and cough can more reliably be suspected of having swine flu.

The new guidelines will likely be to treat without necessarily testing for swine flu.

"There are ongoing discussions at the CDC now as to when exactly that might be," he said.

The government has the power to regulate distribution of drugs in the face of a public health emergency, said Stubbs of the American College of Physicians, but so far that has not been necessary.

Drug wholesalers say they have not been instructed to alter their supply practices, though they are staying in close touch with the CDC and public health officials.

Cardinal, which supplies the leading drug store chains CVS Caremark Corp and Walgreen Co, said it operates a "fair share" policy under which customers receive product based on their historical buying patterns. They are allowed to order more, but only up to a certain percentage.

That is not enough for some customers.

"We've had a number of stores that are now very low on supply or are awaiting more supply, but at this point I cannot speculate on when they might get it," said Tiffani Washington, a spokeswoman for Walgreen. "The situation is evolving right now so I don't even have a figure on how many stores have it or don't have it."

Tamiflu is made by Roche Holding AG while Relenza is made by GlaxoSmithKline Plc. Glaxo said on Friday it is boosting production of Relenza and preparing to start manufacturing a pandemic vaccine.

In the meantime, Stubbs said people should not panic. "Even though this is a novel genetic virus, it seems to be something people can fight."

(Additional reporting by Lewis Krauskopf in New York; Editing by Tim Dobbyn)

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