[photochemsyn]

The concept of addiction gets floated around a lot but I think it's important to distinguish between actual biochemical addiction and what's probably better described as obsessive-compulsive reward-seeking behavior.

Biochemical addiction seems to be characterized mainly by the alteration of the cell-surface concentration of receptor proteins for various endogenous substances as well as lower baseline biochemical production of endogenous substances, and is most well known and studied in opiate addiction. Notably not all of what are commonly considered 'drugs' result in such forms of physical addiction (characterized by physiological withdrawal with specific defined features, often life-threatening if not managed by a physician).

The other version, obsessive-compulsive reward-seeking behavior, has so many modes that it's hard to even characterize simply. Sometimes society rewards such behavior, i.e. working 80-hour weeks even if that negatively impacts your personal health and leads to a breakdown at some point in the future. Sometime society derides such behavior, i.e. playing video games 24-7 (with no compensation) or eating compulsively and becoming overweight. Frantically hunting karma points on Reddit might fit this profile as well as things like gambling habits. Regardless this kind of thing is more of a poorly-understood psychological issue, and lumping it in with physical addiction doesn't seem to make a whole lot of sense.

To be honest I don't know which category caffeine fits in; even after drinking many espresso shots per day I'd find that stopping for a week had little physical effect as long as I drank lots of water, and perhaps took an aspirin or ibuprofen at times. This could vary from person to person however; some people appear to metabolize caffeine at much higher rates than others do.