I think my "problem" may be that when I write, I tend to start out in a de-pressed state, at -50 on the emometer. When I finish a piece, or achieve a goal, it increases to zero, which is a normal, relaxed-happy state. Is your initial value a zero?
Endorphin and adrenaline rushes can definitely cause addiction or addiction-like experiences. But, and this is just a personal observation that may not be generally applicable, after I defeated the alcohol (in a cold turkey process that was nothing short of traumatic, though not without insights that I'm grateful for, at least in retrospect), I spent the next two or three years being overly worried about potential replacement drugs (and they did exist: caffeine, sugar, World of Warcraft, all at different stages). Everything seemed dangerous and potentially addictive, and I didn't trust myself. There was a lot of insecurity and self-doubt.
This is probably normal, especially if the previous drug experience cut deeply, but the danger is that the fear prevents you from living and enjoying life. I think an important qualifier with addictions is the motivation, the reason why you do something. I didn't drink alcohol because it tasted well or because it gave me a sense of accomplishment, but for the physical effects. Are you writing for the sake of writing, for the fun of it, because you must, or are you doing it solely for the rush? If it's the latter, then it could be a drug. But you're allowed to feel good about finishing something, about having successfully wrestled the beast. Feeling well is the natural state.
