One in seven Americans under age 65 went without prescribed medicines in 2007 as drug costs spiraled upward in the United States, a nonprofit research group said on Thursday.

The Center for Studying Health System Change in Washington, D.C., studied the issue, and Laurie E. Felland, a senior health researcher at the center and lead author of the study, said a number of factors contributed to the trend, including rising drug prices, the tendency of physicians to prescribe drugs more frequently, the introduction of expensive new specialty medications, and skimpier drug coverage that shifts a greater share of costs onto patients.