Medicare’s new prescription drug coverage should help most elderly, including some of the poorest beneficiaries, save money, but nearly a quarter will pay more for medicine, according to an analysis released on Monday.

The Kaiser Family Foundation estimates 18.6 million elderly Medicare participants - roughly two-thirds of the 29 million expected to sign up for the benefit - will save money under the plan approved by Congress in December.

The foundation estimates an average savings of 37 percent for all those enrolled beginning in 2006, when the permanent program replaces a temporary drug savings card that was launched in June.

But the savings varied greatly by income level. About 8.7 million elderly who have the lowest incomes and earn extra subsidies will save about 83 percent on out-of-pocket drug costs, the study said.

Roughly 7.4 million participants will pay an average of $492 more a year, researchers said. Another 3 million are expected to spend no money on medicines in 2006.

Kaiser’s study “shows the law mostly ended up being a low-income benefit,” said Barbara Kennelly, head of the consumer group National Committee to Preserve Social Security and Medicare.

The study was based on Congressional Budget Office (CBO) estimates for the new benefit.

Diane Rowland, the foundation’s vice president, said the coverage should work as it was designed by law, but added it was not clear if it would be enough to meet expectations of older Americans.

It is also possible that fewer Medicare participants will sign up for the benefit than the CBO anticipates.

Kennelly said: “It’s going to be difficult to sign these people up” because many seniors are already confused by the temporary drug discount cards.

Officials for the agency that runs Medicare said some of Kaiser’s data was “flawed,” adding it did not include other coverage options from state programs, supplemental insurance and retiree plans.

The benefit’s aim is to “provide key support and ... a foundation for even more comprehensive coverage,” not to be the sole provider, said Mark McClellan, head of the Centers for Medicare and Medicaid Services.

Kaiser’s data also did not factor in other Medicare-related costs to consumers, including higher overall yearly dues, which jumped by 17.4 percent for next year.

The elderly also will have to pay for all prescriptions not covered by the drug plan they choose and for any medicine that falls into the so-called “doughnut hole” - a gap in coverage after patients have accrued $2,250 in costs but before $5,100. After that, the government pays most of the drug costs.