HELENA — The $400 billion Medicare prescription drug bill that President Bush signed into law Monday could end up costing Montana and other states money as the federal government reaches out to states to help subsidize the new program.

A ‘‘clawback'' provision in the bill allows the federal government to demand back almost all the prescription drug savings that states will realize under the bill, federal policy analysts said Tuesday.

Add in the administrative costs that states will bear to support the new program, and states will be digging deeper into their shrinking coffers to come up with money for the plan, analysts say.

‘‘It's not necessarily a boon to states,'' said Trudi Matthews, chief health policy analyst at The Council of State Governments in Lexington, Ky., a nonpartisan association of state government leaders.

‘‘States will get very little relief from this bill,'' Matthews said. ‘‘And they're going to have significant new administrative responsibilities.''

Jeff Buska, Montana's senior Medicaid analyst, emphasized that Montana officials don't yet know how the prescription drug benefit is going to affect the state budget. The federal Center for Medicare and Medicaid Services has yet to write the rules and regulations, he said.

But Buska predicts that Montana will have to kick back $10.4 million to the federal government under the ‘‘clawback'' provision when the prescription benefit begins in 2006.

The ‘‘clawback'' allows the federal government to recoup the savings states will see when they are no longer obligated to buy prescription drugs for those low-income Medicare recipients who already qualify for the state-funded drug benefit under Medicaid. Medicare is 100 percent federally funded while Medicaid, a program for the poor, gets 27 percent of its funding from the state.

Since Montana paid $11.6 million in fiscal year 2003 for the prescription drugs of people who are ‘‘dual eligibles'' — people who qualify for both Medicare and Medicaid — it seemed that the state stood to save that much money when the federal program assumed the full costs of their prescription drugs.

However, the ‘‘clawback'' will take 90 percent of those savings in 2006, Buska said, or about $10.4 million.

By 2014, the ‘‘clawback'' provision will drop to 75 percent of estimated state savings. However, those estimated savings will be adjusted for inflation as the years pass.

The state has no say in how much it must pay to the federal government, and if states don't pay, they'll be penalized.

‘‘If we don't have any control over expenditures of the drugs, and the federal government is identifying inflation rates, that would be a concern to the state,'' Buska said.

Buska went on to say that the state will have to spend more money on administrative services, but he has no idea how much. The state will need to transmit more data to the Center for Medicare and Medicaid Services as client eligibility is determined.

He also said the new benefit could persuade more people to sign up for Medicare.

‘‘There is going to be administrative impact,'' Buska said. ‘‘The devil's in the details and we don't know what they are yet.''

Matthews said states can expect to shell out more money in the first years of the program.

‘‘It's not going to be a fiscal windfall by any means,'' Matthews said.