[dom3k]

Caffeine is a psychoactive substance of the stimulant class. You could definitely categorize it as a "drug".

[esperent]

I know, I'm taking issue with calling sugar a drug, not caffeine.

[pixelpoet]

If I told you a consumable substance is mind-altering, habit-forming, pleasurable and difficult to quit, very bad for your health in the quantities most abusers take it, but they continue to do so anyway- what would you call it?

I've always maintained that most "drugs" are just drugs other people dislike, and everyone is apparently happy to go along with this cognitive dissonance; hence the common phrase "drugs and alcohol", "drunk driving" vs "drug driving" etc etc.