sorry for the miscue on the abuse thing...I get it now. You are talking about substance ABUSE vs. substance addiction.
This is just my 2 cents and not an expert view or anything, but my exah claims he is not an addict because he is not 'physically' addicted, meaning he doesn't think he has withdrawls when he quits using (which I would beg to differ because he would complain of muscle pain, etc, when he didn't have oxy), but be that as it may......
IMHO, anytime drug or alcohol use is bad enough to cause problems in family relationships, jobs, etc., then a line has been crossed. In my situation, I couldn't care less at this point what name a doctor or counselor pins on what my exah has done. The bottom line is that his use has caused multiple serious problems with myself and my children, and he refuses to see that or accept any responsiblity for it.
So the only thing I could do is set boundaries and seperate us from his use, and thus from him.
I'm not sure a therapist or doctor can really tell the difference anyway; the people who live with the person in question are much more qualified to determine whether or not the use is serious enough to cause problems.