Eddie vedder quote. what does he mean?

Question

"Caffeine. the gateaway drug"

Answer 1

he is not saying coffee is as bad as pot.
he is saying that self-medicating with caffeine will lead you into other addictions.  And lest we forget that fragile humans can form addictions to just about anything in attempts of avoidance(s*x, abuse, shopping, food, pain, drama, therapy), eddie is stating something very basic and universal, and quite profound.