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The Centers for Medicare & Medicaid Services said that because actual drug costs were lower than expected, Part D drug plan sponsors will need to return $4 billion to the agency.

Higher generic drugs use among participants and drug plan bids based on incorrect information about drug utilization and enrollment numbers likely contributed to lower actual costs, CMS said.

The Part D program is 30% less expensive overall than originally estimated when the benefit was created by the Medicare Prescription Drug, Improvement and Modernization Act of 2003, according to CMS. The act requires CMS to pay plan sponsors prospectively, based on their bids. It also directs the agency to reconcile accounts after the end of the calendar year.