Max Mushrooms is an item in the “Stat boosts” category (Battle Items). It costs ₽6000.
In-game descriptions
| Sword / Shield | Mushrooms that have the power of changing Dynamax forms. They boost all stats of a Pokémon during battle. |
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Where to find it
Not placed on the atlas maps (so far: Red/Blue and Generation 3).
Sources : PokeAPI (items, item_flavor_text, machines) · atlas · pokeemerald species_info.h