Whenever this topic comes up, I point out that addiction is merely an undesirable side effect. A drug can be extremely enjoyable, but not cause a compulsion to take it again and again. And conversely, a drug can be neutral happiness-wise, but cause an addiction. We all know examples of similar things in real life: a creative flow state is very enjoyable but hard to get into, and Pringles are kind of disgusting but you don't want to stop until you've finished the pack.

My term for this is "the want-like distinction", and the best reference is Yvain's "Are Wireheads Happy"? http://lesswrong.com/lw/1lb/are_wireheads_happy/