Greg, interesting comment on Vietnam and those soldiers who came back addicted to heroin.  Studies done on wartime addiction at the time showed an interesting trend and that is once the soldiers came home a lot of them lost their heroin addictions. You may recall in that era that heroin was the drug of choice then.  This was not expected and it was then learned how important environment was to introducing addictions and maintaining them. This was a big change from what was known about addiction then.  Simple it seems that once out of the stressful life and death situation of being in an active war zone was over those who were using heroin in the war zone did not need to use it anymore.