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Regeneration, otherwise called HP Regen or FP Regen, is a positive status effect that appears in Paper Mario: The Thousand-Year Door, Super Paper Mario, and Paper Mario: Sticker Star. Due to the obvious lack of FP in Super Paper Mario and Sticker Star, there is only the HP Regen from its' debut in The Thousand-Year Door.
Paper Mario: The Thousand-Year Door
The Slow Shroom is capable of granting Mario, his ally, or enemies (assuming they're holding the items and use them) the HP Regen statuses, 2 HP for 10 turns, respectively. They can also be given through Zess T.'s recipes, such as the Meteor Meal (which not only restores 2 HP for 3 turns, but heals 7 HP as well), Couple's Cake (2 HP for 10 turns, akin to the Slow Shroom), and the Shroom Broth (which heals 15 HP, and restores 2 HP for 5 turns, with the downside being Mario or his partner's Defense being lowered). The Happy Heart and Happy Heart P badges function something like this as Mario or his partner gets healed for 1 HP at the end of each turn. More of these badges increases the healing factor at the end of each turn. The Lucky Start badge can also grant the HP Regen status.
The Gradual Syrup, just like the aforementioned Slow Shroom, gives Mario, his partners, or enemies who use the Gradual Syrup, the FP Regen for 10 turns, healing 2 FP at the end of each turn. The Lucky Start badge is the only other way to get the FP Regen. The Happy Flower works exactly like the Happy Heart badge, instead it heals 1 FP at the end of each turn. More of these badges can increase the regeneration output.
Super Paper Mario
The FP Regen status does not exist due to the fact there is no FP used in the game. That means the Gradual Syrup has a new function from its' previous appearance: it gradually restores 20 HP over time. The Gradual Syrup, rather than being a separate item, is now made from a Peachy Peach and a Long-Last Shake. The Long-Last Shake can regenerate up to 10 HP at minimum, 20 at maximum, depending on how well the Action Command had been performed. The Dayzee Syrup doubles the player's attack for 40 seconds, gradually regenerates 13 HP, and awards Mario and the Heroes of Light 1000 points. The Heartful Cake, instead of decreasing Defense and healing 20 FP, gradually restores 9 HP. The Sap Syrup halves damage done for 40 seconds and restores 13 HP. Finally, the Stamina Juice gradually restores 13 HP.
Paper Mario: Sticker Star
The Big 1-UP and Big Shiny 1-UP Stickers restore HP at the end of each turn, just like in The Thousand-Year Door.