From the Super Mario Wiki
A Poison Mushroom (or Poison Shroom) is a harmful variety of Mushroom. Usually, they have the opposite effects a Super Mushroom may have, and while they are sometimes called a Power-Up, they act more like a hindrance or trap. They first appeared in the Japan-only game, Super Mario Bros.: The Lost Levels, which first came to North America on the SNES in the game Super Mario All-Stars, alongside Super Mario Bros., Super Mario Bros. 2, and Super Mario Bros. 3, and have appeared in a number of games since then.
Super Mario series
Super Mario Bros.: The Lost Levels
In Super Mario Bros.: The Lost Levels, a Poison Mushroom will defeat Small Mario or Luigi, or revert Super or Fire forms into small; they have no effect on the Invincible form or the small form that is still flickering from a recent injury. Another way of getting rid of one is to reveal another power-up, such as a Super Mushroom or a 1-Up Mushroom, as this causes the first power-up to disappear. Poison Mushrooms are found in Brick Blocks and ? Blocks, coming in many different colors in the original FDS game. In the Super Mario All-Stars remake, Poison Mushrooms changed their main appearance to blue, due to graphical changes. In Super Mario Bros. Deluxe, their sprites changed once again, this time looking like a pink mushroom with a green cap that has a skull on it.
Super Mario 3D Land
After 25 years of absence in the Mario platformer games, Poison Mushrooms reappear in the Nintendo 3DS game Super Mario 3D Land. In this game, they somewhat behave like enemies as they follow Mario or Luigi around until he is in an area that the Poison Mushroom is unable to get him, or falls into a bottomless pit. The player is also able to destroy them by launching a fireball or boomerang at them, or transforming into a statue to make them vanish on contact. Their appearance is that of a purple mushroom with pink spots that emit a purple dust (possibly spores) when released and grabbed. They appear only in the Special Worlds, and their effects are the same as in Super Mario Bros.: The Lost Levels, where Mario or Luigi can lose a life if they grab one in their small form.
Mario Kart series
Super Mario Kart
Poison Mushrooms were also featured in Super Mario Kart as items, however they were only used by the CPU-controlled drivers Toad and the Princess, and not usable by the player. If a player drove into a Poison Mushroom, they would become small, ending up moving slow and vulnerable to other karts in the area, as they could end up squashed if run over by a full-sized kart. An interesting feature of the Poison Mushroom in Super Mario Kart is the fact the player can also become big again if they run into another Poison Mushroom; this also works if the player were made small from a Thunderbolt by the other player (as CPU players cannot use a Thunderbolt in this game).
Mario Kart Arcade series
In both Mario Kart Arcade GP and its sequel Mario Kart Arcade GP 2, the Poison Mushroom returns as one of Toad's special character items. Like in Super Mario Kart, Toad can use the mushroom to his advantage by causing his opponents to shrink if they come in contact with his poison mushrooms. However, unlike in Super Mario Kart, Toad can now throw the Poison Mushrooms either in front of him as well as behind him (providing him with advantages in both ways).
Poison Mushrooms were featured in the Game Boy version of Donkey Kong. Sometimes, Donkey Kong or Donkey Kong Jr. would throw them at Mario, and sometimes they were contained inside barrels. Whenever Mario comes in contact with one of them, it will cause him to briefly shrink in size, disabling some of his abilities in the process (the ability to swim, climb ladders, etc.). After a few seconds, Mario grows back to his regular size.
Super Mario RPG: Legend of the Seven Stars
The bad Mushroom, only found in Seaside Town, does not recover HP and is used only in battle. When used, they poison an enemy of choice, but some enemies don't get poisoned, so it is not that useful. They have the same appearance as a regular Mushroom in the game.
Mario Party series
Mario Party 3
In Mario Party 3, the Poison Mushroom is an item that the players can use on either themselves or one of their opponents. A player who is cursed by a Poison Mushroom can only roll a 1, 2, or 3 on their next turn. It is possible to eliminate the curse by using another Mushroom or a Magic Lamp. If the player's character is still under effect of the mushroom, Tumble will say that they are still sick from it. It is Waluigi's favorite item.
Mario Party 5
In Mario Party 5, the Poison Mushroom could be found as a capsule. If a character lands on a space where it has been placed or if the player uses it on themself, they can only roll the die with numbers from 1 to 5.
Mario Party 6
The Poison Mushroom returns in Mario Party 6, with a small change. It reduces the dice block numbers from 1-6 to 1-3 and is exclusive to Solo Mode.
Mario Party 7
Luigi's MansionLuigi's Mansion there are Poison Mushrooms that the portrait ghosts throw when Luigi tries to suck them up. They make Luigi small, and also makes the ghost escape as Luigi is too tiny to use his Poltergust 3000. Also, if Luigi calls for Mario by pressing the button while small, his voice is high in pitch.
Super Smash Bros. series
Poison Mushrooms appear as items in the Super Smash Bros. series starting with Super Smash Bros. Melee. They have a similar appearance to Super Mushrooms, though Poison Mushrooms are duller in color and feature angry eyes. They are grabbed upon running into them, and they make the player smaller and easier to KO. In Super Smash Bros. Brawl and Super Smash Bros. for Nintendo 3DS / Wii U, Poison Mushrooms also have the added effect of weakening the player's attacks. They also appear in the Subspace Emissary in some stages.
Mario & Luigi series
Mario & Luigi: Superstar Saga
In Mario & Luigi: Superstar Saga, one of the things Trunkle can suck in his mouth during the "Envelop and Heal" move is a Poison Shroom. They are colored the same, and Poison a brother if touched. Popple sometimes throws them during his "Throw from the Bag" move; these Poison Shrooms cause more severe damage when touched. When the brothers travel to Little Fungitown, Mario eats an Invincishroom, a mix of a Starman and a 1-Up Mushroom. However, Mario, being an outsider, was not immune to the dubious concoction and fell gravely ill. This Invincishroom was, to Mario, like a Poison Mushroom.
Mario & Luigi: Partners in Time
In Mario & Luigi: Partners in Time, a Poison Shroom can be an item used by the two Dr. Shroobs and also Swiggler in the battle against him. The Shroobs also resemble poison mushrooms. In addition, one of Shrooboid Brat's attacks involves spitting either Mushrooms or Poison Mushrooms at Mario and Luigi, though they are launched as the opposite of what he is given (if he is given a Mushroom, it is spit out as a Poison Mushroom, and vice versa). Like in the previous game, getting hit by a Poison Mushroom inflicts the brothers with the Poison status effect.
Paper Mario series
Paper Mario: The Thousand-Year Door
The mushroom was seen once again in Paper Mario: The Thousand-Year Door. Poison Shrooms could be created by Zess T. by mixing a Slow Shroom and Inky Sauce, Point Swap or Trial Stew and a Dried Bouquet. If Mario or his partner eats this mushroom, the user will lose half his HP and be afflicted with the Poison status. In battle, there is a small chance that when eaten, the Poison Shroom won't poison its user, but will restore all HP instead. There is a common misconception that the Feeling Fine badge(s) are required for this to happen, but it is simply a random occurrence. They're one of the pictures that the player can earn during a Bingo! (and unlike the items, are drawn with faces). If three in a row appear, Mario's HP, FP and SP will all be cut in half and all of the audience members will leave.
Super Paper Mario
In Super Paper Mario, Poison Shrooms are a much more common occurrence than in its predecessor, occasionally being dropped by Zombie Shrooms, Cursyas, and their relatives. They are used slightly more in recipes; for example, a single Poison Shroom can produce a Dangerous Delight. In addition, any recipe that requires a Poison Shroom can be cooked using a Ghost Shroom instead, if Poison Shrooms are in short supply (Ghost Shrooms can be bought from various shops, or obtained via Merlee's charms).
Paper Mario: Sticker Star
In Paper Mario: Sticker Star, the Poison Mushroom returns as a sticker, having the same appearance as in Super Mario 3D Land. It poisons Mario upon usage; however, the effect will also transmit to enemies that attack him in close combat while it is active. Another one of its effects is that it prevents Mario from wearing defense or evasion stickers such as the Tail, the Shiny Tail, the Super Boot, the Shiny Super Boot, the Frog Suit, and the Shiny Frog Suit.
It is also used as a 'key' to open the door in World 2-1 to enter the desert. Putting one inside of a Paperization Block will yield three Shiny Leaf stickers. A Toad in a back alley of Decalburg will sell the player a heavily overpriced Poison Mushroom (only once Hither Thither Hill is unlocked), but the player can negotiate to get the price down to 25 coins.
Mario Hoops 3-on-3
In Mario Hoops 3-on-3, Poison Mushrooms appear in Power-up panels. They make the player weak and slower.
Super Smash Bros. Melee
*Note: This is incorrect. Poison Mushrooms first appeared in Super Mario Bros.: The Lost Levels, but as this was only released in Japan (as "Super Mario Bros. 2"), its inclusion in the compilation game Super Mario All-Stars was the first time it appeared in any other region. In addition, they only cost characters a life if they were small; this implies that they do no matter what the circumstances are.
Super Smash Bros. Brawl
Super Smash Bros. for Nintendo 3DS
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