The Tasty Tonic is an item appearing in the first two Paper Mario games. It raises Mario's HP and fixes/cures multiple Status Effects, usually shrinking or poison. It also one of the more common drops by enemies after a battle.You can feed this to Luigi and he will give you an Ultra Shroom as well as all of the Star Pieces. They are sold in Paper Mario for 5 Coins, and 3 in Paper Mario: The Thousand-Year Door. Tasty Tonics are also recipes, and can be cooked with the following ingredients:

Bubble Berry
Honey Syrup and Yellow Berry
Honey Syrup and Blue Berry
Honey Syrup and Red Berry
Honey Syrup and Apple
Honey Syrup and Lime
Honey Syrup and Lemon
Honey Syrup and Coconut