Murder Riddles

Two girls; one lives, other dies

Helen and Louise, two high school girls, around 16, go out for a well deserved girls night.
It's a sweltering hot night, so they decide to go to an out door restaurant.
They sit outside the restaurant, listening to live music and telling each other stories.
When the waiter comes, to take their order, they both ask for two ice teas, "with extra ice!" They said in unison, laughing happily afterwards.
When their drinks arrived, Helen gulped down hers, seeing she was so thirsty.
But Louise, being a save-it-later type of person, only finished when the waiter arrived again sigh the bill.
After a lot of arguing of who was going to pay, Louise wins, and slides her card through the machine.
She is about to enter her PIN number, when she slumps to the table, dead.
Panic arises in the restaurant, and the manager calls the paramedics.
Later they conclude, that she had been poisoned, and that Helen had taken the exact same amount of poison, but yet it is still unclear, why she had lived, and not Louise.

Why did Helen survive, but Louise died?
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