Lemonade is, in addition to be being a sour juice, a healing item which can be found in all main Pokémon games. In Red and Blue and FireRed and LeafGreen, it is always on the top floor of the Celadon Department Store, in the vending machines; in Gold, Silver and Crystal, the top floor of the Goldenrod Department Store, in the vending machines, and in Pokémon Ruby and Sapphire it can be found in the vending machines of the top floor of the Lilycove Department Store. It can also be bought in bulk on Two Island in FireRed and LeafGreen once you've gotten the National Pokédex. It costs 350 and sells for 175.
It recovers 80 HP in health of the Pokémon it is used on. In addition to this, it is also a key item to get past the Saffron guards in Red, Blue and Yellow; in FireRed and LeafGreen, the item to get past the guards was replaced with Tea.