Congress could eliminate the Part D “protected classes” rule which forces insurers to pay for any drugs in six arbitrary categories, regardless of their price or value. (The Trump administration had proposed just such a reform, but withdrew it in a sop to the drug lobby.)
More substantially, Congress should require that drug companies selling drugs into Part D rebate any price increases above consumer inflation to Medicare, to offset the program’s taxpayer-funded subsidies.
Without reforms of this type, it’s highly likely that restructuring Part D would drive costs upward.
