Researchers from the University of Chicago Medical Center, Harvard University in Boston and Virginia Commonwealth University in Richmond used data from more than 117,000 patients ages 66 to 79 to assess the impact of the new prescription benefit plan.

The analysis, published online ahead of the print issue of the Annals of Internal Medicine, found the new benefit led to a 13.1 percent decrease in out-of-pocket expenses for seniors and a 5.9 percent increase in prescription use.

Study author G. Caleb Alexander of the University of Chicago Medical Center said the researchers compared out-of-pocket costs and the number of pills purchased by those who were eligible for Part D with comparable patients who were not.

"Despite extensive debate, it was not clear to what extent Part D would save people money or allow them to obtain drugs they might not otherwise be able to afford," Alexander said in a statement. "We found that it had a modest but significant effect on both savings and drug use."