has anyone done this? i want to get some to use w/my soda stream so i can make caffeinated seltzer water, i'm sick of drinking dehydrating coffees and teas in the afternoon and can't tolerate store-bought energy drinks any more as they are too sweet. 
problem is FDA says this is illegal 
internet says buy preworkout (? wouldn't this have other stuff in it) and i've also found this which seems maybe interesting but expensive.
my dream beverage and what i hope to achieve, is something that's 24oz, tastes like a LaCroix, and has like 300mg of caffeine in it lol.
suggestions?
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Be careful with pure caffeine, that shit could kill you no joke.
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