A look at Medicare's top 20 priciest prescription drugs in 2015, ranked by their cost above the program's "catastrophic" coverage threshold. Medicare's catastrophic protection kicks in after a beneficiary has spent a given amount of their own money, $4,850 this year. The beneficiary pays only 5 percent, while their insurer pays 15 percent, and taxpayers cover 80 percent. Catastrophic spending is a large and growing share of total costs, threatening to make Medicare's popular prescription plan financially unsustainable.

Some medications have additional uses.

Cost above catastrophic threshold in 2015; includes spending by taxpayers, insurers and beneficiaries.

Source: Centers for Medicare and Medicaid Services, Office of the Actuary