Poison Mushroom

A Poison Mushroom is a mushroom that kills Mario or Luigi if they touch it. Poison Mushrooms appear in Super Mario: The Lost Levels.

Darker red Poison Mushrooms were later seen in the Super Smash Bros. series, in which it temporarily shrinks characters down to one-fourth their original size.

The purple top with the skull was changed after Super Mario: The Lost Levels to a simple dark green mushroom.

Paper Mario: The Thousand-Year Door
The mushroom was seen once again in Paper Mario: The Thousand-Year Door. Poison Mushrooms could be created by Zess T. by mixing a Slow Shroom and Inky Sauce or Trial Stew and a Dried Bouquet. If Mario or his partner eat this mushroom, the user will lose 17 HP and be poisoned.

However, in battle, if you have the Feeling Fine (P) badge equipped when using a Poison Mushroom, there's a chance the user's HP will go to max.