While the U.S. Congress scrambles to fix Medicare's strategic growth rate-related cutbacks, the drug component's premiums have risen by as much as 16 percent in the past year for new enrollees in its ten most popular prescription plans, according to a June report written by healthcare research firm Avalere Health.

The average premium paid in May 2008 for the top ten Medicare Part D drug plans was $26.39, up from $22.80 in July 2007. For the top three plans, this difference was starker, with average May 2008 premiums of $28.15 and a July 2007 average of $22.57.

"Medicare beneficiaries are conscious of value, and many switched into lower cost plans for 2008," Penny Mills, Avalere vice president, says in a statement. According to the report, average premiums would have grown by about 21 percent, had beneficiaries remained in the ten most popular plans of 2007.

But not all the top ten raised their rates. SilverScript lowered its monthly rate about 30 percent to $20.84 in 2008 from $27.40 in 2007, while expanding the drugs covered on its formulary, notes Avalere. However, increases were common, with Humana PDP Standard, AARP MedicareRx Saver and WellCare Classic each raising premiums by more than 50 percent, the company adds.

The ten most popular Part D plans controlled 72 percent of the market in April 2008, a share that shrunk from 75 percent in June 2007.