Trips to the pharmacy are getting more and more expensive. From 2006 to 2013, the average retail cost of a year’s supply of 622 common prescriptions doubled—from about $5,500 to more than $11,000—according to a new report from AARP, a senior citizen advocacy group.

The rising costs are likely to hit senior citizens hardest, the AARP cautioned, noting that the 2013 average cost would come out to about 75 percent of the average Social Security retirement benefit, which is $15,526.

“If these trends continue, more and more Americans will simply be unable to afford the medications that they need to get and stay healthy,” Debra Whitman, AARP’s Chief Public Policy Officer, said in a statement.

In their report, the group tracked the retail prices of 227 brand name, 115 specialty, and 280 generic prescription drugs between 2005 and 2013. While the prices of the drugs increased every year, the amount of each year’s increase has also been steadily rising. In 2006, the average cost uptick from 2005 prices was 3.6 percent. But by 2013, prices shot up an average of 9.4 percent over those from 2012.

The inflation rate, on the other hand, has seen no such rise. In 2013, there was only a 1.5 percent lift from 2012.

While the price of generic drugs actually fell by an average of 4 percent between 2012 and 2013, the rising cost of specialty and brand name drugs outweighed the savings. Brand name drugs saw the steepest rise—12.9 percent. Specialty drugs went up by an average of 10.6 percent.

“Unfortunately, it’s becoming clear that we can no longer rely on decreases in generic drug prices to offset unrelenting price increases for brand name and specialty drugs,” Leigh Purvis, director of health services research in AARP's Public Policy Institute, said in a statement. “This shift has serious implications for older adults and the entire health care system.”

In an August survey by the Kaiser Family Foundation, 24 percent of Americans overall reported struggling to pay for their medications. Of those in poor health, 43 percent reported having trouble.

The report warned that continued and accelerating drug price increases could also lead to ballooning health insurance premiums, out-of-pocket costs, and potentially higher taxes and cuts to government programs as more tax dollars go to cover the drug bills.