Washington ? President Bush signed a new prescription drug benefit into law Monday as part of historic Medicare changes that also will confront seniors with numerous, sometimes-difficult choices on their health care coverage.

Bush said the new drug insurance “will save our seniors from a lot of worry.” But the bill’s critics said the worries had just begun for Medicare’s 40 million older and disabled Americans.

The government will spend nearly $400 billion in the next 10 years to subsidize prescription drug coverage, which begins in January 2006. At the same time, the government will encourage insurance companies to offer private plans to millions of older Americans who now receive health care benefits under terms fixed by the federal government.

“Medicine has changed but Medicare has not — until today,” Bush said, explaining that prescription drugs and outpatient care had replaced hospital stays in the past two decades. “Our seniors are fully capable of making health care choices, and this bill allows them to do that.”

The first tangible result of the Medicare law will be prescription drug discount cards that the president said would take effect in June. He said seniors would receive a mailing in the spring to explain the card, which will cost no more than $30 a year. It will offer discounts that Bush said would range from 10 percent to 25 percent off retail prices. Critics say the promise of savings is wildly inflated.