Wow that is a good question and one I am not qualified for, but it has always been my understanding that your an addict if your actions have a negative affect on your life and the life of the ones around you.
The only example I can give you to this would be my son who is always telling me he doesn't understand why we don't just leave him alone "he is not hurting anyone, but himself" which is not true, he is stealing my things to support his drug use, he is not obeying the house rules and he is not the same kind, good hearted person he was before using drugs. I struggle with what you are asking also, If my son is clean a few days I question myself with "well maybe he is not an addict he just likes to use them here and there" but he always goes back to the drugs and each time it is a little worse. One other note that has proven to be true is addiction is a progressive disease.....so with time he/she will use more and more.