It is shaping up to be the biggest shift yet to a generic drug, potentially saving the nation $2 billion a year or more in prescription costs.

And scientists and doctors say that for most of the 16 million people in America who take drugs to reduce cholesterol, the low-priced alternative will work as well as the name-brand medicine — Lipitor, which is made by Pfizer and is the nation’s most widely prescribed drug.

While Lipitor itself is not available as a generic, a very similar drug made by Merck, Zocor, lost its patent protection last year. The generic version of Zocor, simvastatin, is now much cheaper than Lipitor, leading insurers to press doctors and patients to switch.

But Pfizer is not letting its flagship drug go down without a fight.

The company has mounted a campaign that includes advertisements, lobbying efforts and a paid speaking tour by a former secretary of the federal Department of Health and Human Services. Pfizer is also promoting a study — whose findings many experts are questioning — that concluded that British patients who switched to simvastatin had more heart attacks and deaths than those who remained on Lipitor.

The Lipitor battle has become a test of the pharmaceutical industry’s ability to defend name brands, even as insurers, patients and doctors seek to whittle the nation’s $270 billion annual prescription drug bill by using generic alternatives whenever possible.

Lipitor and other cholesterol-lowering drugs, sometimes called statins, are the largest drug class, with spending of $22 billion last year in the United States alone. And they have been researched more thoroughly than any other group of drugs, making head-to-head comparisons easier.

Many doctors have come to see simvastatin as a viable substitute for Lipitor. Studies show that at commonly prescribed doses Lipitor and simvastatin are equally effective at reducing LDL cholesterol, the so-called bad cholesterol.

A big difference is that Lipitor costs $2.50 to $3 a day, while simvastatin sells for 75 cents to $1 a day at most retail pharmacies, and as little as 10 cents a day at discount pharmacies like Costco’s.

Each month, doctors with patients on Lipitor are switching tens of thousands of them to simvastatin. And simvastatin is also taking a growing share of the market for new patients who need a cholesterol drug. “Simvastatin is much less expensive to society over all and to patients,” said Dr. Thomas H. Lee Jr., a prominent cardiologist. “If you put patients on generics,” he said, “the chances that they’re taking their medications six months later are higher than on a brand name drug. I think that a few hundred dollars a year does matter.”

But Pfizer argues that Lipitor is the most effective statin and that patients who are having good results with it are not well-served by moving to another drug.

“The only reason one would want to switch from one drug to another is for the benefit of the patient’s health,” said Dr. Michael Berelowitz, senior vice president for worldwide medical affairs for Pfizer.

In September Pfizer began sounding safety alarms by citing an analysis of the medical histories of 2,500 people in Britain who switched to simvastatin from Lipitor, compared with 9,000 who did not make the change. The study concluded that patients who switched were more likely to have a heart attack or stroke than those who remained on Lipitor.

The results were presented on a poster at a European cardiology conference. And Dr. Berelowitz said the study had been accepted by the British Journal of Cardiology and would soon be published.

But independent researchers say that limitations in the study, which was conducted by Pfizer’s own researchers, gives it little predictive power about what will happen to patients who take simvastatin instead of Lipitor. And they say the study is far less important than large clinical trials that have shown simvastatin’s effectiveness at reducing cholesterol.

“It will run counter to everything that’s been published to date if it’s true,” Dr. Lee said of the Pfizer study. He is president of the network of about 5,000 doctors in Partners HealthCare, the health system formed by Massachusetts General Hospital and Brigham and Women’s Hospital in Boston.

Dr. Mark Fendrick, a professor of internal medicine at the University of Michigan and a specialist in health care economics, notes that for patients with extremely high cholesterol, Lipitor may be a better choice. An 80-milligram daily dose of Lipitor, the top dose, can reduce cholesterol by up to 60 percent, compared with about 50 percent for an 80-milligram dose of simvastatin, also the top dose.

But most patients with moderately high cholesterol take 10 or 20 milligrams of Lipitor a day, and can get comparable benefit from 40 or 80 milligrams of simvastatin, Dr. Fendrick said.

Dr. Robert O. Bonow, the chief of cardiology at Northwestern Memorial Hospital in Chicago and a past president of the American Heart Association, said patients’ cholesterol levels should be monitored after the change, to make sure the simvastatin is having the desired effect.

“Switching itself is not a problem,” Dr. Bonow said. “It’s not that one drug has more risk or less risk.”

Lipitor’s share of the cholesterol-lowering drug market in this country has ebbed to 30 percent, down from 40 percent 18 months ago, when simvastatin was available only as name-brand Zocor — at prices that were higher than Lipitor’s.

No generic version of Lipitor is in the offing because the Lipitor patent remains valid until at least March 2010. But the advent of generic Zocor has dented sales enough to hurt Pfizer’s stock, which is trading near its lowest level in a decade.

In a recent conference call with Wall Street analysts, Pfizer vowed to step up its efforts to protect Lipitor. So far this year, the company has been spending more than 50 percent more on advertising the drug than it did in 2006, when its Lipitor ad spending for the year totaled $142.7 million.

Lately, Pfizer has been running a print and broadcast advertising campaign that features Dr. Robert Jarvik, the inventor of the artificial heart, endorsing Lipitor.

“There’s a common misconception that all cholesterol-lowering medications are the same,” Dr. Jarvik says in a radio ad. “They’re not. There is no generic version of Lipitor.”