Chain drugstores argue that their business model is different from a company like Costco or Sam's Club. CVS, Rite Aid and Walgreen Co. all say that about two-thirds of their revenues come from prescription drugs.

"We don't sell snow tires," says Mike DeAngelis , a spokesman for CVS/pharmacy. "The core of our business is the pharmacy."

Mr. DeAngelis wouldn't say how much CVS pays for generics but confirmed profit margins on generics are generally bigger than for branded drugs. On all drugs, Mr. DeAngelis says, CVS's prescription-drug profit margins are 2% to 3%. A spokeswoman for Walgreen gave a similar figure, and a spokeswoman for Rite Aid said the company's prescription-drug margins were very thin.

Walgreen spokeswoman Tiffani Bruce says there can be a lag in retail price reductions if the company had stocked up on large quantities of a generic before the manufacturers' prices dropped. She also says profit margins on generics help the pharmacy chain make up for much smaller profits -- or even losses -- on branded drugs. The beginning of a generic's availability "represents a small window where drugstores are able to recover from the weight of carrying all these unprofitable prescriptions," Ms. Bruce says.

Patients whose drugs are covered by insurance often don't notice subtle shifts in generics prices, because insurers typically allow patients to pay the lowest co-payments for generics as soon as they are available. As big purchasers, insurance companies can often make steep cuts in what they will pay for widely used generic drugs.

Insurer Humana Inc., Louisville, Ky., is paying pharmacies about $20.50 for the typical amount of simvastatin, according to the insurer's Web site. UnitedHealthcare is paying a typical amount of $38.20, its site says. Both insurers say they pay pharmacies more for drugs than it costs the pharmacies to acquire them.

For patients who aren't covered, online price searches can be helpful. But the sites have limitations. For instance, if you don't purchase the drugs via a chain's online outlet, prices at the bricks-and-mortar locations may vary. Mr. DeAngelis of CVS says individual stores' prices may be a little higher than the Web site's. CVS's site doesn't list prices for generic drugs but provides a customer-service number to call for that information.

Other sites that offer price comparisons include destinationrx.com. More tips can be found on the Consumer Reports site crbestbuydrugs.org. The National Center for Policy Analysis, a think tank that promotes "consumer driven" health care, offers tips such as bulk-buying and pill-splitting in a publication on its site ncpa.org, called "Shopping for Drugs: 2007." Drugstore.com, for instance, says consumers can spend less per pill by buying larger quantities.

A spokeswoman for Rite Aid says the chain will match local competitors' prices if a customer asks. In cities, the competitors would likely have to be located "within blocks," she says. In rural areas, the distance could be five miles or so.

Jacob Goldstein contributed to this article.