According to a recent report,

... 29% of the nation’s prescription drug spending was for Medicare beneficiaries in 2014, compared to 18% in 2006, the first year of the Part D benefit.

Source: "10 Essential Facts About Medicare and Prescription Drug Spending," The Henry J. Kaiser Family Foundation, July 7, 2016, http://kff.org/infographic/10-essential-facts-about-medicare-and-prescription-drug-spending/