WASHINGTON (AP) — A safeguard for Medicare beneficiaries has become a way for drugmakers to get paid billions of dollars for pricey medications at taxpayer expense, government numbers show. 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, according to the program's number-crunching Office of the Actuary. 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. The pharmaceutical industry questions the numbers, saying they overstate costs because they don't factor in manufacturer rebates. Medicare's catastrophic coverage was originally designed to protect seniors with multiple chronic conditions from the cumulatively high costs of taking many different pills. The presidential candidates, as well as the Obama administration, have proposed giving Medicare legal authority to negotiate prices. The drug industry says Medicare patients are getting valuable, innovative medicines. Concerns about catastrophic costs undercut the image of Medicare's prescription program as a competitive marketplace in which private insurers bargain with drugmakers to drive down prices.