The way caffeine works is weird. It's less that it makes you energetic and more it slots into the receptors in your brain for another chemical that tells it that you're tired. The brain adapts so it makes more receptors so you have to consume more caffeine for the same effects. Mezz could have built a tolerance over time.
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Oh believe me, I know about tolerance. I'm talking about something a little different though: the way the body responds, even completely fresh to a given substance, long before any tolerance has had time to build up. Personally, I'm kind of both: I'm VERY sensitive to caffeine, to the point that going in fresh after months away produces an almost euphoric effect, far beyond what most people describe. But on the other hand, while tolerance does build eventually and repeated use ends up negatively impacting my mood, even on a fresh exposure I'm never really "jittery" like some people describe, twitching and shaking, feeling like the world's closing in on me etc. There are a lot of people that basically give themselves panic attacks if they have any significant amount, and I've never felt even remotely like that. The worst caffeine has ever done to me is like how Bilbo described the effect of the One Ring: like butter spread over too much bread.