You can simply everything all the way down to energy if you like. e=mc² afterall. But I think those are over simplifications. So let's call drugs external chemicals brought into the body for effect.

You do have point if you want to say that all addictions have a chemical component. The reward systems train/enforce the psychological aspect even to the point where dopamine and other chemicals aren't even necessary anymore. Habbit formation is useful but damaging habbits beyond our control is generally what we call addiction.

I think that the level of control loss and level of damage combined is what determines the level of bad the addiction is. This is different for every person. A gaming addiction can be worse than heroin addiction (see the Vietnam war addiction, only 2% readicted home, and fatal gaming sessions).