A Poison Mushroom is an exclusive Item for two select racers in Super Mario Kart- Princess and Toad. However, it is only available for CPU controlled characters; otherwise, you get the regular Question Blocks.
If you hit a Poison Mushroom, you get shrunken down to a very small size, similar to the Lightning Bolt, which causes you to lose speed. Another kart can then proceed to drive over you to flatten you, causing you to lose even more time. This danger makes it very important to get an antidote. The first is to sacrifice yourself and get crushed, but a more reasonable way is to hit another Poison Mushroom... it returns you to normal somehow. And yes, they can also return you to normal size after a Lightning strike.
Mario Kart Trivia
- A Mega Mushroom has nearly the same coloration as a Poison Mushroom, with the yellow cap and red spots, and yet they have the complete opposite effect.