I guess I would want to know the intention behind the question before I answered. Is the OP an addict looking for hope? Is the op wanting to help an addict looking for hope? Or is the op looking for someone to agree that addicts can't possibly be a believer, so they can go back with more ammunition against addicts?
No one is perfect, everyone has to make their own choices in life and live with the consequences of ALL of those choices, whether good or bad, and sometimes we all make bad choices that lead us down the rabbit hole. The odd thing....God loves us all regardless of our choices. When an addict cries for help with anything, God acts.....just like when a saint asks for help God acts. So whether someone is a "Christian" or not, isn't really my business. If they ask me for help, I will act. I don't need to know their heart, because God does, and I trust God.
Peace!!