While I realize that mental illness is series (experienced it in my family) it seems to me the bipolar diagnosis is a bit overused. It seems that the mood swings caused by meth use accounts for it.
What I have seen it gives people the sense that they have an excuse for throwing temper tantrums whenever they like.
I'm not a mental health professional, but I am an attorney. And any attorney who has some experience practicing criminal law, builds up at least a little subject-matter knowledge on the drug/crazy connection. If there are some therapists reading this, perhaps they add some actual expertise, but....
I think there is a nexus between some drug use (meth especially) and bipolar disorder, and that connection goes both ways - sometimes the bipolar disorder is the rootcause of the meth addiction, and other times the meth has so damaged the neural pathways, bipolar disorder (and other mental illnesses) is the end result of the meth use. IOW, just stopping the meth use won't necessarily "cure" what's ailing the addict. The damage well indeed be permanent, and have to be managed for the remainder of the addict's life.