According to ThinkProgress:

Drug offenses remained the single most common cause of arrest in 2012, mostly for offenses involving mere possession, according to newly released FBI estimates. Of the 12.2 million estimated arrests 1.55 million were for “drug abuse violations.” Some 82 percent of those were for possession offenses, and 42.4 percent for marijuana possession. That is the equivalent of a drug arrest every 20 seconds, and a marijuana arrest every 42 seconds, according to calculations by Law Enforcement Against Prohibition, a group of law enforcement officials who support the regulated legalization of drugs.

Also, North Country Public Radio produced a very interesting report discussing why some black leaders were initially supportive of the war on drugs. It worth listening to here.