The government will not be able to sustain the costs of the Medicare Part D prescription drug benefit, according to the head of the Government Accountability Office.

The benefit is “financially irresponsible,” U.S. Comptroller General David Walker said on the CBS television program “60 Minutes” on Sunday. The Medicare program will need $8 trillion to cover the difference between what it will take in and what it will owe beneficiaries over the next 75 years. A portion of the Medicare Part D costs go toward dual eligibles, or those receive Medicare and Medicaid. Many nursing home residents fall into this category.

But acting CMS Administrator Leslie Norwalk said the drug benefit is necessary. “Over 90% [of Medicare beneficiaries] are covered and the program costs are much less than what experts predicted when the bill was enacted in 2004,” she said.