Red Water of Life

In LoZ, Link can buy 2nd Potions from the medicine shops in Hyrule. 2nd Potions cost 68 rupees and contains 2 doses of medicine. After Link uses one, it turns into a Life Potion with 1 remaining dose. Each dose recovers all of Link's hearts. Before he can buy 2nd Potions, he needs to find the Letter. Once Link finds the Letter, he gets access to both Life Potions and 2nd Potions.