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It’s just tea with sugar.
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So, are you “allowed” to do it? Sure.
Is it “good”? it’s just added sugar so probably not.
Will it “kill you”? Only to the extent that massively over-consuming those things might kill you anyway. If you just have gallons and gallons of sugary Yerba Mate and forced fed it to yourself, you’d die from caffeine overdose long before sugar from what you’re describing.