From the Super Mario Wiki
A Tasty Tonic is an item in Paper Mario. The item is sold all over the world but is least expensive at the Item Shop in Yoshi's Village for the price of two coins each.
A lot of Tasty Tonics are available throughout the game, in most item stores. Although not a rare item, these items can be useful.
A Tasty Tonic cures any Poison and/or shrinking done to Mario.
The Tasty Tonic reappears in Paper Mario: The Thousand-Year Door and serves the same purpose. It is sold in the item shop in Rogueport for three coins; it is also sold on the Excess Express for three coins.
Tasty Tonics were taken out of Super Paper Mario, because there is no 'Shrinking' in this game and a normal HP recovery item will cure poisoning.
In Paper Mario, Tasty Tonics can be used outside of battle. Since all status ailments go away at the end of a battle this has no effect.
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