For a person who does believe in sin, traditionally, it has always been considered one of the seven deadly sins. Addiction, I suppose could be considered gluttony, but at some point an addict loses control of his/her desire and understanding of what their addiction is and is doing to them. In order for something to truly be considered a sin, the person committing the act must be completely and fully aware that it is sinful.
Gluttony by definition is basically continuing to eat (or consume) after you have enough, and then to continue, and continue, and continue...etc. Overeating is a very good example. For example those individuals who get up into the 600lb bracket or more and just continue to eat and eat.
I hope that answer was helpful.