A safeguard for Medicare beneficiaries has become a way for drugmakers to get paid billions of dollars for pricey medications at taxpayer expense. The cost of Medicare's catastrophic" prescription coverage jumped by 85 percent in three years, from $27.7 billion in 2013 to $51.3 billion in 2015. Out of some 2,750 drugs covered by Medicare's Part D benefit, two pills for hepatitis C infection - Harvoni and Sovaldi - accounted for nearly $7.5 billion in catastrophic drug costs in 2015. Medicare's catastrophic coverage was originally designed to protect seniors with multiple chronic conditions from the cumulatively high costs of taking many different pills. Beneficiaries pay 5 percent after they have spent $4,850 of their own money. With some drugs now costing more than $1,000 per pill, that threshold can be crossed quickly. Lawmakers who created Part D in 2003 also hoped added protection would entice insurers to participate in the program. Medicare pays 80 percent of the cost of drugs above a catastrophic threshold that combines spending by the beneficiary and the insurer. That means taxpayers, not insurers, bear the exposure for the most expensive patients. Catastrophic spending accounts for a fast-growing share of Medicare's drug costs, which totaled nearly $137 billion in 2015. The catastrophic share was 37 percent, yet only about 9 percent of beneficiaries reached the threshold for such costs. Catastrophic coverage will soon cost as much as the entire prescription program did when it launched. Experts say the rapid rise in spending for pricey drugs threatens to make the popular prescription benefit financially unsustainable.