The agent for the addiction only becomes a problem when injested. The problem is why take a substance to get a meaningful life or to run away from a meaningless life.
Somebody should do research on the purchasing of drugs by the prison system. That initself is a rich source of information and speculation as to the history of addictions in this nation.
The article linked below has more links embeded in it that will make for good reading and fuel all kinds of speculation. Also, there are some film clips of individual stories that some will resonate with.