The great promise of cheap generic drugs is taking a bumpy road to arrival.

Case in point: Zocor, one of the most commonly prescribed pills in the U.S., which lost patent protection last June. Multitudes of patients have switched to generic versions of the cholesterol-lowering drug, lured by lower insurance co-payments or the promise of a significant price drop for those who pay out-of-pocket.

As predicted, the price that many insurers pay for generic Zocor has dropped dramatically. But the price that pharmacies charge patients who pay cash remains high in many locations, with wide variations by vendor. At online pharmacy walgreens.com, for instance, the price for 30 tablets of a 20-milligram dose of Merck & Co.'s Zocor is $149.99, compared with $89.99 for simvastatin, the generic version. And last week, the same dose of simvastatin cost $108.99 at CVS 's Web site, compared with $154.99 for Zocor. After a call from a reporter, CVS said it would drop its simvastatin price to $79.99, as part of an "ongoing price analysis."

At a time when policy makers are searching for ways to cut health-care costs, generic drugs are often viewed as one of the most straightforward solutions. But as the situation with generic Zocor illustrates, prices can vary wildly, and may not be nearly as cheap as expected. Generics of a number of other notable drugs that came off patent recently -- including the antidepressant Zoloft, the antibiotic Zithromax and allergy drug Flonase -- have also so far failed to deliver big savings in many cases.

"We're not seeing that sharp a drop-off" in price among generic drugs that have come out in the past couple of years, says Jim Yocum , executive vice president of DestinationRx Inc., a Los Angeles pharmacy data and software company. "We're just not seeing it."

To be sure, even for the uninsured, generics still typically cost less than their branded counterparts. And at big clubs such as Costco Wholesale and Sam's Club, out-of-pocket prices for generics do generally plummet. Simvastatin costs $6.97 for 30 pills of the 20-milligram dose at a Sam's Club for which the company provided price information.

But just how far -- and how fast -- generic prices fall depends on a number of factors. Among them: how many generics makers sell the drug; how much competitive pressure pharmacies feel; whether there is another alternative, such as a different generic in the same class of drugs; and whether a particular generics maker gets an initial exclusivity period. By law, the first generics maker to challenge a patent on a branded drug and prevail wins six months of exclusive sales.

For the more than 46 million Americans without health insurance -- plus perhaps millions more whose insurance plans don't cover drugs -- the lesson is to shop carefully. Some pharmacies list prices online, and certain Web sites will compare prices from a number of competitors. If your pharmacy doesn't list prices, a few phone calls to some competitors can mean big savings. If you're willing to travel, try pharmacies that aren't very close to each other and are less likely to react directly to each other's prices.

The growing variety of outlets for prescription drugs -- with wholesalers and online vendors joining the chains and mom-and-pops -- brings a range of business models that affect how generics are priced. At sellers where drugs are a significant driver of revenues, prices may remain high. Sometimes, as with generic Zocor, prices stay high even after a six-month exclusivity period ends, when additional generics makers start fueling supply.

Stores say they regularly review prices. At Drugstore.com Inc., generic simvastatin until recently had been at $125 for the common 30-tablet dose, compared with $135.99 for Zocor, even after the six-month exclusivity period ended in late December. After a reporter called to inquire about the price, drugstore.com on Friday dropped simvastatin to $27.99, which the company said was part of a regular review. Zocor now costs $139.99. On walgreens.com, simvastatin's price hadn't fallen after the six-month period's end. After a reporter inquired about it in late February, it dropped to $89.99 from $129.99. A spokeswoman said the price had already been under review.

At Rite Aid Corp. pharmacies, a spokeswoman last week said the average price of the common Zocor dose was $178.99, compared with $131.99 for simvastatin. Yesterday, the spokeswoman said those prices were inaccurate and that the company frequently adjusts its prices, keeping them in line with competitors; she declined to provide new figures, saying it was proprietary information.

Prices can also vary widely for other generics. Pfizer Inc.'s antidepressant Zoloft lost patent protection last summer, and generic Zoloft's six-month exclusivity period ended in early February. At drugstore.com, branded Zoloft costs $87.14 for 30 tablets at 50 milligrams, compared with $67.99 for the generic, sertraline. The generic is priced similarly high compared with branded Zoloft at cvs.com and walgreens.com, and somewhat less at a Wal-Mart location. But at the Sam's Club location, the generic is $5.97.

Data through January from Wolters Kluwer Health show the average retail price that consumers paid for the generic of nasal spray Flonase has hardly dropped since it became available about a year ago. A common dosage and package size of the generic of antibiotic Cipro cost consumers nearly as much as branded Cipro in mid-2003, then fell sharply around mid-2004, according to Wolters Kluwer Health. And the average retail price consumers paid for a common dosage and package size of the generic of antidepressant Paxil held steady for about three-quarters of a year until mid-2004, then fell steadily until around the fall of 2005 -- when it increased before falling again.

At Costco Wholesale, whose Web site yesterday listed the common dose of generic Zocor at $11.96, the company says that even at such prices, Costco is making a profit. Charles Burnett , senior vice president of pharmacy at Costco, says the company can acquire the 30-tablet, 20-milligram dose of simvastatin for $2.71. He says the price on costco.com today will fall to $10.66. Patients are allowed to use the pharmacies of clubs such as Costco and Sam's Club, even if they aren't members.