First it is important to note that there are no external causes of addiction. There are reasons that individuals internally decide to use heavily (living in poverty might be one of them) – but there is no external factor that causes one to use beyond their control. This is important to keep in mind when looking at why people like to use substances. This topic is addressed in detail in The Freedom Model for Addictions. Here is a brief section from Chapter 5 on the research that refutes poverty as an inevitable cause for addiction:

“If we told you putting a lit match to an open tank of gasoline “causes” a fiery explosion, you could test this claim. Assuming you survived the explosion, you could do it a hundred times, and it will always result in an explosion. But if we told you that poverty causes addiction and you went to a poor neighborhood to survey 100 people, you might find somewhere between 5 and 20 people who currently fit the diagnosis of addiction. Why weren’t the other 80 people caused to use substances heavily? Then you could go to a high-priced treatment center and find nothing but people who grew up in wealth and luxury. What caused them to become addicted if not poverty? Or say we told you trauma causes addiction. If you rounded up 100 people with high trauma scores, only 15 of them might also be alcoholics. This is what the research shows. Yet people confidently claim that trauma causes addiction, as if the individual has no choice in the matter, and is simply fated to use substances relentlessly after they’ve been hurt. In fact, this response to trauma is the exception, not the rule—the other 85 are not “addicted.” Are they superheroes with magical powers somehow able to flout the law that trauma causes addiction?”