Medicare Part D is a federal program administered through private insurance companies. These companies offer retail prescription drug coverage to Medicare beneficiaries. Medicare Part D is simply insurance for your medication needs. You pay a monthly premium to an insurance carrier for your Part D plan. In return, you use the insurance carrier’s network of pharmacies to purchase your prescription medications. Instead of paying full price, you will pay a copay or percentage of the drug’s cost. The insurance company will pay the rest.