Caffeine is serious business - in the 1950s, people would get something that was called "coffee nerves" then. Near-overdose on caffeine from too much coffee. Jittery, irritable, insomnia type stuff.
Then people started using it directly as a substitute for benzedrine - which had actually been legal (!) - (long haul truckers, etc.) When I traveled for business in the '80s and '90s, I preferred to drive, and the rest stops and diners (oh, diners!) all sold various caffeine pills for the same purpose. I lived in Rochester NY for awhile, home of Jolt cola "all the sugar, twice the caffeine" and it was quite effective when I worked overtime. Also tasted pretty good, at least to me, at least back then .
Now it's probably energy drinks, which are like turbocharged Jolt. I've never tasted one - Jolt was quite enough for me
People can actually overdose on these and die - sadly it's often younger people with cardiac conduction defects or other undetected issues that make them vulnerable. With coffee, even with espressos, you usually get really uncomfortable before you're in serious danger, and that will slow or stop your consumption.
I guess if there's a moral to this bit of rambling it's that caffeine is a respectworthy stimulant, and we forget that because it's part of our routine diet... but it can have strong positive or negative effects.
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"I believe you find life such a problem because you think there are the good people and the bad people," said the man. "You're wrong, of course. There are, always and only, the bad people, but some of them are on opposite sides."
-- Terry Pratchett, Guards! Guards!