CHICAGO – In an aggressive move to win over consumers that also threatens to shake up the $252 billion pharmaceutical business, Wal-Mart Stores Inc. said Thursday it plans to test the selling of many generic prescription drugs for the unusually low price of $4 per month.

Wal-Mart, the world’s largest retailer, already has remade the retailing landscape across the nation, where it has become the biggest seller of apparel, groceries and toys. But it is hungry for more growth and sees a chance to bring its extraordinary size and clout to bear on rising health care costs.

Health care is also a politically charged issue for Wal-Mart, which has been criticized for not offering enough insurance to its employees. That prompted some analysts to question whether the company’s motive for promoting discounted generic drugs is tied to efforts to polish its corporate image.

The test offer is scheduled to begin Friday in Tampa, Fla., followed by an expansion across the state in January. Wal-Mart said it intends to roll out the program to the rest of the nation starting next year.

The $4 price for a wide selection of drugs would be available to everyone, regardless of whether they have health insurance.

The announcement roiled stocks of rival pharmacies, sending shares of Walgreen and CVS Corp. plummeting more than 7 percent each – the biggest one-day decline for each company in more than five years.

Both companies count on prescription drugs for more than two-thirds of their total sales and have been relying on generic drugs for profit growth. Generic drugs are big money makers with profit margins of 30 percent to 60 percent.

For retailers, selling prescription drugs drives customer traffic, which is why they typically station the pharmacist at the rear of the store, hoping to spark sales of other merchandise as shoppers come and go. Wal-Mart has been suffering from slower sales growth and is under pressure from Wall Street to find ways to boost revenue.

“Looking over the decades, in various moments of American capitalism, certain industries and companies are on top and they really have market power,” said Nelson Lichtenstein, history professor at University of California, Santa Barbara, who studies Wal-Mart’s impact on the economy. “We had the railroads and Microsoft, now it’s Wal-Mart. They’ve used their clout against pickle manufacturers and garment makers. What about pharmaceuticals?”

It’s a good time for Wal-Mart to expand its pharmacy business because as the baby boom population ages, prescription drug sales are poised to soar in the coming decades. Additionally, the market for generics is exploding as scores of brand-name drugs lose patent protection over the next five years.

All told, Wal-Mart will sell about 120 different drugs in various dosages for $4. For people with insurance, that price tag translates into a savings of the co-payment of $5 to $13 a month, according to Todd Swim, a Chicago-based principal with Mercer Human Resource Consulting. For the uninsured, savings could be substantially higher.

Under the program, consumers will have access to a wide array of generics to treat common conditions such as Metformin for diabetes, Albuterol for asthma and Lisinopril for high blood pressure.

Still, skeptics warn the list of drugs will do little to provide relief to consumers using the most widely prescribed expensive drugs. Most of the drugs on Wal-Mart’s list are older generics that are relatively inexpensive already, said Stephn Schondelmeyer, professor of pharmaceutical economics at the University of Minnesota.

For example, not on the list are generic equivalents of such blockbuster drugs as the cholesterol pill Zocor and the antidepressant Zoloft, both of which lost patent protection this year.

“This isn’t addressing what is the source of people’s frustrations with drug prices,” said Schondelmeyer. “This is a very limited set of drugs. It’s a lot of hype and will create a lot of traffic going into Wal-Mart stores. But I think people are going to be disappointed when they go into the stores and find out their drug isn’t there.”

At the moment, Wal-Mart is a minor player in the retail drug business. Standard & Poor’s food and drug retail analyst Joseph Agnese estimates all the mass merchants combined, including Wal-Mart and Target Corp., account for less than 10 percent of prescription drug sales in the U.S.

Walgreen, for its part, doubted the impact on consumers because many of the generics on the list were older. “When you dig into this, it’s not going to have nearly the impact the headlines would suggest,” said Walgreen spokesman Michael Polzin. “We are doubtful that this will drive individual behavior.”

CVS officials didn’t return calls seeking comment.