A new study has revealed that the percentage of users of prescription drugs in the U.S. rose from 51 percent of U.S. adults in 1999 to 59 percent of adults in 2011, Reuters reports.

The report was published in the Journal of the American Medical Association.

Lead author Elizabeth D. Kantor, formerly of the Harvard T.H. Chan School of Public Health in Boston said,

"For example, we know that older adults tend to take more medications than younger adults, and so we'd expect prescription drug use to increase as the U.S. population ages,"

But "something beyond the aging of the U.S. population appears to be driving the increase in prescription drug use," Kantor said.

She noted that several factors were responsible for the rise in the number of prescription drug users. These include the old drugs losing patent protection and becoming less costly when new drugs enter the market and the patterns of prescription drug use evolving with advances in science and changes in clinical guidelines and policies regarding drug marketing and promotion.

"There's so much going on in each area, it's hard to draw concrete conclusions," Kantor said, according to Washington Post.

"Each drug class stands on its own."

According to Washington post, obesity was one of the likely factors driving the increased use. A frequently prescribed drug treats gastroesophageal reflux, a common condition among the overweight or obese.

Researchers also noted that eight of the 10 commonly used drugs in the United States were for hypertension, heart failure, diabetes and other elements of "cardiometabolic syndrome."

A sharper increase for several drug classes was witnessed early in the study period, with the trend slowing down in more recent years, Kantor said.

For the study, Kantor's team studied seven cycles of the National Health and Nutrition Examination Survey (NHANES) including 37,000 U.S. adults age 20 and older.

The study noted that the increase in prescription drug use was only significant for people age 40 and older. With the increase in the use of prescription drugs, the use of five or more drugs at once, called "polypharmacy" also increased from 8 percent to 15 percent of adults.