The Centers for Disease Control and Prevention (CDC) released a study that found that the number of Americans taking at least one prescription drug monthly rose from 44 percent to 48 percent between 1999 and 2009. 

From the report:

In the United States, spending for prescription drugs was $234.1 billion in 2008, which was more than double what was spent in 1999 (1). As new drugs are introduced and new uses for old drugs are found, more patients can have improved health and quality of life with the appropriate use of prescription drugs. Current prescription drug use patterns need to be better understood. This report provides an overview of current prescription drug use in the United States, how many and what kinds of drugs are currently being prescribed, and who receives them.

Full Report: Prescription Drug Use Continues to Increase:U.S. Prescription Drug Data for 2007–2008 (PDF | 658 KB) 

Centers for Disease Control and Prevention. (2010). Prescription drug use continues to increase: U.S. prescription drug data for 2007-2008. Qiuping, G., Dillon, C., and Burt, V.