There was a significant increase in overall prescription drug use and polypharmacy among US adults between 1999 and 2012.
In this nationally representative survey, significant increases in overall prescription drug use and polypharmacy were observed. These increases persisted after accounting for changes in the age distribution of the population. The prevalence of prescription drug use increased in the majority of, but not all, drug classes.
Kantor et al. (2015) studied this question.