Washington ? Medicare officials on Friday unveiled the final design for a prescription drug benefit that will take effect Jan. 1, promising savings of hundreds of dollars for a typical middle-class beneficiary.

But officials acknowledged they face widespread confusion and skepticism among retirees about how the voluntary plan would work — and even whether it is worthwhile.

Medicare Administrator Mark McClellan said a $300 million campaign is being launched to educate 42 million elderly and disabled beneficiaries about their options. About 6 million beneficiaries who have very low incomes or are in nursing homes will be enrolled automatically.

The success of the effort should play a large part in determining whether the new benefit — a major Bush initiative that the president spotlighted during his re-election campaign — becomes a valuable addition to the bedrock health program or turns into a politically embarrassing flop.

Much of the concern about the drug plan stems from its differences with existing Medicare benefits. Unlike those, it won’t be provided automatically to the majority of beneficiaries.

Instead, most retirees will be required to make a series of decisions, starting with whether they want to participate at all. Coverage will be provided through private plans, requiring beneficiaries to shop around.