About 12 percent of the 41 million current Medicare beneficiaries are in such private health plans today.
The number of Medicare beneficiaries has been growing about 2 percent a year.
More than a third of Medicare beneficiaries have to pay for all drugs themselves.
Medicare beneficiaries now spend about $8 billion a year on such drugs, according to Government data.
The drug benefit is available to all 42 million Medicare beneficiaries.
A third of the 40 million Medicare beneficiaries have no insurance to cover drugs.
About 14 percent of the 39 million Medicare beneficiaries are now in managed care plans.
Under the president's plan, some Medicare beneficiaries would shoulder added costs.
In fact, Medicare beneficiaries have many more choices than officials had expected.
In the last three years, private health plans have dropped more than 1.6 million Medicare beneficiaries.