Poison Mushrooms (also called Cursed Mushrooms) are mushrooms similar in appearance to a Super Mushroom and a 1-Up Mushroom, except of a slightly darker shade. Touching it in any game reduces the character's size (similar to the Mini Mushroom) or reverts the effects of any power-ups, and can also take off a player's life if he is small. In some games the Poison Mushroom looks similar to the Super Mushroom but they have the opposite effect and sometimes the Poison Mushroom will have "angry" eyes and a purple cap with a skull symbol instead of spots, such as in Super Mario Bros.: The Lost Levels. Sometimes, the poison mushroom will be purple, making is easier to spot and avoid.
Poison Mushrooms are found in a number of 2D Mario games, as well as the Super Smash Bros. series, in which the character's size is reduced, but not the size of the character's attacks. The Poison Mushroom will also make the character lighter, allowing their opponents to easily knock them out of the stage.
Poison Mushrooms also appear in Super Mario 3D Land, They only appear in special worlds. where they have a purple cap without a skull, and angry eyes. They emit a trail of purple gas, which is just for show. They steer themselves towards Mario, but if Mario reaches a place where the Poison Mushroom can't get at him, it will disappear.
Poison Mushrooms also appear in Paper Mario: The Thousand-Year Door, but not as items. When in battle, there is a chance that poison mushrooms will appear on the Bingo! wheel. If you match up three poison mushrooms, you and your partner will lose half your HP, and you will also lose half your FP and SP.
Poison Mushrooms then come in Super Paper Mario with a purple top without the skull and with angry eyes. When you first get it is's a regular mushroom, then it turns into a poison mushroom. It's mouth will go up and down constantly.
Poison Mushrooms return in Paper Mario: Sticker Star in the form of sticker. The Poison Mushroom poisons both Mario and enemies for two turns. However, they are useless against poison-based enemies such as Poison Bloopers.
It also appears in Super Mario Maker 2 as a replacement of 1-UP Mushroms in the Night Ground theme. They can climb up walls in that game.
In the Mario Party series
The Poison Mushroom is a common item that appears in the Mario Party series. The Poison Mushroom is usually depicted with a dark blue-purplish color, with a skull cap and an angry face on the mushroom, like in Super Mario Bros: The Lost Levels. When a player receives a Poison Mushroom, their numbers on the Dice Block are usually reduced.
The Poison Mushroom debuts in Mario Party of this series. They are referred to as Poisonous Mushrooms and can only be obtained as one of two Mushrooms of the Mushroom Space (the other being the Super Mushroom). If a player is unfortunate enough to get a Poison Mushroom, they will lose the next turn.
The Poison Mushroom returns in Mario Party 2. In the board map Mystery Land, a player has a chance to summon a "curse" on another character by a Shy Guy. The character that receives the curse will have a Poison Mushroom placed on their head. When their next turn began, the high number they could get on the dice block is a three. The Poison Mushroom first appearance as an item was in Mario Party 3. In this game, a player could buy a Poison Mushroom from Baby Bowser's Item Shop for five coins, or win it in a Item Mini-game. When a player used a Poison Mushroom, their dice block was cut to only the numbers 1-3. The Poison Mushroom was very useful for landing on desired spaces that were only a few spaces away.
In Mario Party 4, the Poison Mushroom was replaced by the Mini Mushroom, which acted similar to the Poison Mushroom. When capsules were introduced in Mario Party 5, the Poison Mushroom was renamed the Cursed Mushroom, but still retained its original appearance. However, this time, the player would roll a 1-5 instead of a 1-3.