an emotional imbalance or ennui. — Shawn
I'm not quite sure what an "emotional imbalance" is. Say more.
Ennui does not seem like a sufficient cause.
Ennui -- listlessness and dissatisfaction arising from a lack of occupation or excitement..
"he succumbed to ennui and despair". The definition (and many synonyms) don't seem to be sufficient to cause a flight to hard drugs for escape: boredom, tedium, listlessness, lethargy, lassitude, languor, restlessness, weariness, sluggishness, enervation, malaise, dissatisfaction, and so on. Someone who is "sluggish" would be more likely to resort to coffee than meth, wouldn't they? (They would if they were good Methodists, but I suppose a lot addicts are not Methodist.)
Despair, though, that seems like a sufficient cause. Despair, anomie, untreated major depression, extreme poverty (not by itself, but in conjunction with other factors), intense loneliness, feeling abandoned, the sense of not having a future worth living for (but not leading to suicide), and so on.
emotional regulation through substances. Another would be simply thrill seeking through drugs. — Shawn
So yes, emotional regulation as you say.
Thrill seeking is probably a driver too -- one that can trap the thrill seeker into coming back rather regularly for more thrills.
Let's not overlook the fact that drugs are not only sought out, they are also pushed. Methamphetamine wasn't called into common usage by thrill seekers always whining about there just not being any exciting drugs around. Meth was introduced to communities across the country by motorcycle gangs (Hells Angels) who had an interest in developing a market. Same thing goes for cocaine and heroin. People in small towns didn't wake up one day and say,
"You know, we need heavy duty uppers and downers here in this fine small town. Let's help our good neighbors out by setting up contacts with a Mexican drug cartel and start a business here."
No. It was the other way around.
Indeed, one could almost say that hard drug producing countries (Myanmar, Afghanistan (opiates), Columbia (cocaine), China and Mexico (fentanyl) are engaged in biowarfare by flooding the United States and Europe (and other places) with drugs whose long term (or in the case of fentanyl--short term) use may result in death or disability.