From the Super Mario Wiki
The Mini Mushroom is a variety of Mushrooms found in the Mushroom Kingdom. Whoever uses it will shrink greatly in size and will be capable of sneaking into smaller areas. Its polar opposite is the Mega Mushroom, which makes the player grow to a massive size.
Mario Party 4
The Mini Mushrooms debut in Mario Party 4 as pink capped mushrooms with white dots and with a pink pacifier below its eyes. It reduces the Dice Block from 1-10 to 1-5 and allows a person under its effects to enter Gates and play Mini Mini-Games. It costs 5 Coins at the Item Shop and it can also be obtained for free at a Mushroom Space. Additionally, it has an alternate and similar effect form called Super Mini Mushroom, colored entirely translucent pink. It has the same effect, but allows the player to use two Dice Blocks, both ranging from 1-5.
Mario Pinball Land
Mini Mushrooms make a reappearance in Mario Pinball Land, where they can let Mario go into small passages and get extra Power Stars. They usually cost ten coins and take on their appearance from Mario Party 4.
Super Mario series
New Super Mario Bros.
The Mini Mushroom reappears in New Super Mario Bros., where it is now smaller, blue, and lacks its pacifier. It shrinks Mario and Luigi, and again gives them the ability to enter tiny Warp Pipes. Mario and Luigi also gain the abilities to perform Floaty Jumps and run across water, due to their light weight. Unfortunately, one hit will cause a Mario brother to lose a life, and ground pounds are required to damage/defeat enemies and bosses. If Mario or Luigi defeats Mummipokey this way, then they will gain access to World 4; doing the same to Petey Piranha lets them go to World 7. It can be found in four levels: 1-4, 2-Castle, 3-A, and 7-A. It can also appear in Roulette Blocks.
New Super Mario Bros. Wii
They appear again in New Super Mario Bros. Wii. They are just like the Mini Mushrooms from the original New Super Mario Bros. However, they are not used as often and are very rarely seen in levels, but they can be found commonly in Toad Houses. The courses World 4-3, World 2-2 and World 9-8 are the only levels in the game to include a Mini Mushroom in-level. Like in the last game, Mini Mushrooms are required to find several Star Coins and secret areas.
New Super Mario Bros. 2
The Mini Mushroom reappears in a few levels in the game New Super Mario Bros. 2, where it works just as it did in past games. They are also given in some red Toad Houses.
New Super Mario Bros. U
Mario & Sonic at the Olympic Winter Games
A Mini Mushroom appears again in the DS version of Mario & Sonic at the Olympic Winter Games. They are found in Polastraits, Icepeak, and Blizland. They allow Mario to become mini so he can cross thin ice without weighing too much to break it. It also allows him to cross narrow surfaces. Mario can use the Mini Mushroom, while Sonic can't. The effect of this mushroom can only wear off if Mario were to grab a normal Mushroom.
Mini Mushrooms also make a brief appearance on small platforms on the ice in Dream Curling, where players must hit a stone across the field to knock down Goombas. Here, they will make the stone much smaller if hit by it. This will make the stone easier to sweep around, but will also make it harder to knock out all the Goombas.
Mario Sports Mix
In Mario Sports Mix, players can use this item to shrink their opponents. While tiny, players can't hold a basketball, dodgeball, or hockey puck, and are unable to do spikes in volleyball. The tiny players are also unable to perform defensive maneuvers in dodgeball, making them major targets for attacks.
Mario Kart Arcade GP DX
Super Smash Bros. series
While Mini Mushrooms themselves don't appear in the Super Smash Bros. series, the Poison Mushroom has a similar effect on characters if they come into contact with one. Like a Mini Mushroom, the Poison Mushroom will cause the character to shrink down, making them harder to hit, but easier to launch due to their light weight.
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