The Bush administration issued a new policy on Wednesday that protects Medicare beneficiaries against the sudden loss of coverage for drugs they are taking under the prescription drug program.

Under the policy, insurers can still change their lists of covered drugs, known as formularies. But if they drop any drugs or impose new restrictions, they must exempt beneficiaries who are now taking those drugs. 

Dr. Mark B. McClellan, administrator of the Centers for Medicare and Medicaid Services, summarized the policy this way: "In general, a plan cannot change your coverage for the drugs you are using during the year. The stability of drug formularies is extremely important for many of our beneficiaries."

The policy addresses one of the chief criticisms of the Medicare drug benefit. Democrats and a few Republicans in Congress had said it was unfair that drug plans could change their formularies at will while most beneficiaries were locked into a drug plan for the full year. This disparity was a major concern for many consumer advocates and for some beneficiaries. 

The new policy says, "No beneficiaries will be subject to a discontinuation or reduction in coverage of the drugs they are currently using," with some limited exceptions.

An insurer could, for example, remove a drug from its formulary if new research showed that the drug was unsafe for some patients, or if a new low-cost generic version of a brand-name product became available.

The Bush administration issued the policy with less than three weeks remaining before the May 15 deadline for people to sign up for Medicare drug coverage. People who miss the deadline will generally not have another opportunity until November and will then face higher premiums as a penalty for late enrollment. People can switch plans at the end of each year without penalty.

Karen M. Ignagni, president of America's Health Insurance Plans, an industry trade group, said her organization supported the new federal requirement even though "it does not reflect common practice in the private sector," where employers provide drug coverage to millions of workers and retirees.

Ms. Ignagni said the federal policy "will have a financial impact," increasing costs for insurers under Medicare. But, she said, "you need to balance that with the goal of providing continuity of drug coverage and peace of mind to beneficiaries."

In most states, 40 or more drug plans are available. They differ in premiums, co-payments, deductibles and other details. Some plans cover fewer than 800 drugs. Some cover more than 1,800.

Insurers have many tools to manage use of prescription drugs. They can, for example, require doctors to get prior approval for prescriptions and can limit the number of pills given to a patient each month.

The policy says that an insurer can remove a drug from its formulary, increase co-payments or impose new restrictions "only if enrollees currently taking the affected drug are exempt from the formulary change for the remainder of the plan year."

Dr. McClellan, the Medicare administrator, said his agency had received 4,600 requests from drug plans that wanted to change their formularies. About 3,100, he said, involve the addition of drugs or relatively minor changes. About 1,500 requests involve more significant changes that will be covered by the new requirement, he said.

In a memorandum being sent to insurers, the Bush administration says the policy is needed to protect beneficiaries against "bait and switch" tactics in the drug benefit program, known as Part D of Medicare. 

"Medicare beneficiaries select Part D plans, in part, based on the formulary that is marketed during annual open enrollment and therefore have a legitimate expectation that they will have continuing access to coverage of the Part D drugs they are using throughout the plan year," the memorandum says.

Nothing in the new policy fills the gap in coverage found in most plans. After incurring $2,250 in drug costs, beneficiaries typically must pay all of the next $2,850, until catastrophic coverage kicks in.

The policy was issued just as Senator Max Baucus of Montana and other Democrats were putting the final touches on a bill to increase protections for Medicare beneficiaries.

Dr. McClellan said insurers had legitimate reasons for changing formularies because "drug therapies are constantly evolving as new drugs are developed and new medical knowledge becomes available."