You use the term "drug of choice," which I find interesting. Is your fiance an alcoholic, a drug addict or possibly both? Whatever his problem is, it is probably the drug where he has no choice. But that is neither here nor there in regards to your question. Pay close attention to what FormerDoormat said. The question is not so much as whether you should get help for him, but for yourself. You can be honest with yourself now and save yourself a lot of grief. He's going to do what he's got to do and there is nothing you can do about it. In fact, the more you involve yourself in his problem, the more you will eventually become bitter, full of self-pity and self-loathing and remorse and all the rest of the evil brood that comes with watching someone slowly die. Cut the string now. It might hurt right now, but you'll be better off and so will he.
Jim