From the Super Mario Wiki
A Slow Shroom is a Mushroom found in Paper Mario: The Thousand-Year Door for GameCube. A Slow Shroom heals two HP every turn for ten turns. Slow Shrooms are sold at the Deepdown Depot for fifteen Coins.
An item similar to the Slow Shroom, the Long-Last Shake, appears in Super Paper Mario. It gradually heals up to 20 HP, depending on how well its Action Command is executed. Also, the Big 1-UP sticker from Paper Mario Sticker Star gives a similar effect as well, healing 10 HP a turn for ten turns.
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