The whole concept of "addiction" is more controversial than it sounds. Is there a difference between really liking something, and being addicted to it? The chemical and the mental side of a person is linked very tightly. Hormones are chemicals which can affect our mood ... and our mood can cause us to release more of certain hormones.
Heroin is often used as an example of physical addiction, however I suspect the psychological reasons that made the addict start using heroin are stronger. An example which is often used is that after Vietnam, the majority of soldiers who become heroin addicts, immediately kicked their addiction.
In my own view, there is no difference between a rabid sports fan, an enthusiastic Trekkie, a smoker, a heroin addict and a person who is asking their doctor for constant Xanax refills. It's all shades of grey.
I think the important thing is to look at how it affects your life. In some countries they provide safe-houses to shoot up in, and it seems to me, that one of the worst thing about being classed "an addict" is the stigma, which prevents a person from interacting with normal people. If one can carry on with life, like Sherlock Holmes, and take heroin, than I don't see the problem. On the other hand, if one isn't considered an addict, just because one is taking huge amounts of aspirin instead of an illegal drug, but their liver starts to fail ... they need to take action.
At the end of the day, "addiction" is just a word we came up with to try and neatly package human behavior. A thorn by any other name would cut as deep.