I am ADD
Editor's nitpick: I hate this phrasing. No one says "I am depression" or "I am herniated L4-L5." (If they do, send them to me and I will throat-punch them. Then they can say "I am contusion.") (Although I am aware people say "I am OCD," and while that's both a commonly used phrasing and also most likely clinically untrue for most of the people who say it, I still don't like it. It represents a slow slide towards imprecise use of language, much like the death of "decimate.")
Apparently clinical studies (as opposed to anecdata) have shown fairly conclusively that people with ADD/ADHD are *vastly* more likely to develop addictions than are people without ADD/ADHD.
Self-medication. See also, massive use of caffeine.
I think I wasn't quite clear in my previous nerdy post. The higher propensity of comorbid addictions in people with ADD/ADHD is not behavioral (as in your example, they self-medicate with caffeine, or because they need an even bigger thrill, they do coke). The scientific evidence is that there is a neurobiological/neurochemical difference going on (it has to do with dopamine, but I am WAY too undereducated to explain it).
I just think that's really fascinating. In the end, of course, it's manifested as behavior, and so one could *call* it behavioral, but it's not behavioral in origin.