From the Super Mario Wiki
The Mega Mushroom is a rare variety of mushroom found in the Mushroom Kingdom. When eaten, they grow their consumer to a massive size, allowing them to walk over and destroy everything in their path, including the largest and most powerful enemies. Mega Mushrooms made their debut in Mario Party 4, where they were depicted as normally proportioned mushrooms with green caps, however their current appearance is quite different and unique amongst the Mushrooms of the Mario series.
Introduced in New Super Mario Bros., the current Mega Mushrooms have bulbous, orange-yellow caps with red spots, and while all other mushrooms slide along the ground, the Mega Mushrooms hop. Originally, they would actively flee from Mario, and if the player jumped over one, it would even change directions, although this trait was omitted from the Mega Mushroom's New Super Mario Bros. 2 appearance. Its opposite counterpart is the Mini Mushroom, which turns Mario into Mini Mario.
Mario Party 4
Mega Mushrooms first appear in Mario Party 4, with a light green cap (resembling a 1-Up Mushroom). It can be obtained by landing on a Mushroom Space, purchasing it from an item shop, getting it from an Item Bag, or winning a mini-game on the board. They let the player roll two Dice Blocks at once, making the player grow to an overwhelmingly gigantic size and flatten other players as they stomp across the board, stealing ten coins from them. However, board events will be bypassed (except for events involving Bowser at Bowser's Gnarly Party). Additionally, a Super Mega Mushroom is available, colored entirely translucent green. If you get the same number on all two Dice Blocks you get 10 coins (but for two "7's" it is 30 coins). The Super Mega Mushroom does the same thing but with three Dice Blocks.
Super Mario series
Super Mario 64 DS
New Super Mario Bros.
Mega Mushrooms later reappear in New Super Mario Bros., where they again make Mario grow to a giant size. In this colossal form, Mario can stomp across the level and crush anything in his path - enemies, blocks, Warp Pipes, and even the Flagpole at the end of the level. When items are crushed by Mario, the bar at the top starts to fill up with yellow. As the yellow line reaches the end of a segment, that segment turns green. After Mario's size reduces again, a 1-Up Mushroom appears for each green segment filled, with a maximum of five 1-Ups. These mushrooms are rarely found, and almost always come from Toad Houses or Red Flying Blocks. Besides the Toad Houses, they only appear in four levels: 1-1, 1-2, 4-4 and 7-5. If Mario or Luigi ground pounds a block containing a Mega Mushroom, it still appears from above. The player is not completely invincible in this form, however; if the player is to jump and land on a hole covered with bricks, the player falls through, thus, losing a life, if the hole is big enough. In other cases, Mario, or Luigi, lose a life if they fall in lava or the purple poison. Additionally, if Mario or Luigi are to do a Ground Pound after eating a Mega Mushroom, a group of Goombas or Coins (or both) falls from the sky.
New Super Mario Bros. 2
The Mega Mushroom returns in New Super Mario Bros. 2 after their absence in New Super Mario Bros. Wii. They retain the same function as in the first New Super Mario Bros. Like in New Super Mario Bros., they are very rare, only appearing by launching fireballs in a black pipe from World 1-2, World 5-6, or World Mushroom-3, a Mega Mushroom will appear. In addition, Mario cannot gain 1-Ups for the destruction he causes. However, he can get 1-Up Mushrooms by knocking the Goal Pole in the big form. This can only be done in World 5-6.
Super Mario 3D World
The Mega Mushroom makes a return in Super Mario 3D World, making it the item's first appearance in a 3D Mario title. It is the same as in the handheld New Super Mario Bros. games. However, if someone ground pounds, he can now create a small wave that breaks all nearby blocks and enemies. As in the other games, Mega characters can destroy Warp Pipes.
Mario Kart Wii
In Mario Kart Wii, the Mega Mushroom is an item obtained from the Item Box. When players use this item, they grow bigger and their voice gets much deeper. This item offers the players the ability to squash other players for a certain amount of time, reducing their speed and making them lose their items. While this item is active, the players receive a speed boost and cannot slow down when going off-road, and they are invincible to all dangerous things, except for the Star, Bullet Bill, and Blooper, giving them the power to destroy any items or any course obstacles, except those that are big and metalic (such as Chain Chomps). If players get hit by a Thunderbolt while they are big, they shrink back to normal size rather than becoming tiny. The effect also ends prematurely if players fall off the course. The Mega Mushroom's theme music from New Super Mario Bros. is played for the duration of the item's effect.
Mario & Luigi: Bowser's Inside Story
In Mario & Luigi: Bowser's Inside Story, Broque Monsieur sometimes attacks by using a Mega Mushroom to make himself giant. Bowser needs to punch his body to avoid the attack. Then Broque Monseiur's head will fall off, still giant, and Bowser needs to punch his head.
Mario & Sonic at the Olympic Winter Games
In Mario & Sonic at the Olympic Winter Games, the Mega Mushroom appears in the Dream Event, Curling Bowling, and Balloon Blast. The actual item does not appear, but there is an Item Box that it could be obtained from. The Mega Mushroom makes the character 2x his or her original size.
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