Actually, the US prison population is declining in both relative and absolute numbers, and has been for the last six years. http://www.bjs.gov/content/pub/pdf/p14.pdf
Wow this data is going down but look at this: Half of males (50%) and more than half of females (59%) in federal prison were serving time for drug offenses on September 30, 2014.
State prisions and local jails have much smaller percentage of drug charges - about 17% in my state and going down.
http://www.doc.sc.gov/pubweb/research/InmatePopulationStatsT...