At this point probably prevention is gone because they're probably already involved with drugs. From our perspective, it would be treatment, bringing them to a place where they have a certain physiological capacity to overcome an addiction and then make free choices about what they want to do.
But it's not just the absence of an addiction; you also need to buttress that with other social supports if you're going to have a successful reintegration plan. It's pretty critical as part of relapse programming to try to keep people out of circumstances where they could fall back into their addiction behaviour with alcohol or drugs.