Magic Mushroom

In Real Life, Mushrooms, toadstools, and fungi comprise a vast array of types and genuses. While a few are cultivated for food, and some for medicinal or religious (or hallucenogenic) purposes, others are so toxic they are outright lethal, so one should never assume that a mushroom found in the wild is safe to eat without consulting a reference book (or better yet, a trained expert who can distinguish between seemingly identical specimens). Sometimes the visible mushrooms are actually just the tip of a proverbial iceberg, with the majority of the organism being a large underground colony of fungus. And the ability of fungi to reproduce via infectious microscopic spores allows them to grow in just about any environment (including inside living bodies), and can make it nearly impossible to get rid of a fungus infestation.

Fictional mushrooms can do all of the above, and more: Eating a mushroom can produce downright magical effects (like making you bigger or smaller, turning you invisible, etc.), and getting infected by mushroom spores can not only make you sick, but physically transform you into a monster, possibly taking over your mind in the process; villains in particular can even weaponize the spores, turning them into a tool for Mind Control or Brainwashing unsuspecting innocents.

Often used by witches, as it's a symbol of their knowledge of the nature around them. What they do with it is another matter. Toadstools appear to be most common in healing potions or poisons, though the last one does not require much magic. Toadstools and brightly colored mushrooms are often hiding places for smaller members of the Fair Folk. In some cases, the fungi, usually mushroom-like, can even become ambulatory. This is usually not a good thing, unless they just want to dance.

See also Fantastic Fruits and Vegetables.

Anime & Manga

 * Ranma ½ had age-altering mushrooms. The height of the mushroom corresponded to the age into which it changed the eater: A 4 cm mushroom makes the eater 4 years old, a 17 cm one 17 years old, etc. Why the mushrooms worked with the metric system is not explained. (Because even nature has made the switch to metric before the USA.)
 * Then there's the mushroom which, immediately after eaten, makes the victim susceptible to a single "post-hypnotic command" which will be performed whenever they hear the same sound as was around when the suggestion was made. (Why someone selling ingredients to a restaurant would be carrying some of those around with him is anyone's guess)
 * The anime also had vaguely explained "Mushrooms of Love" that caused people who ate some to fall into a state of love that could only be cured by the local "Mushroom God", an extremely large mushroom with healing properties that also included curing mushroom poisoning. If they didn't eat some soon enough (Ranma and Akane, the victims in the episode, had until the first evening star came out), the effect would become permanent. It's never made clear exactly what caused specific couples to form from eating the mushrooms.
 * In One Piece, Luffy almost dies after eating an odd mushroom that causes mushrooms to sprout rapidly from all over his body.
 * Django, once a pirate under Kuro, has the power of hypnosis because he ate a Mushroom. The downside is, now he has a stem of the mushroom growing from his chin like a beard. Oh, and he also tends to fall asleep whenever he hypnotizes someone. But that one seems to be no fault of the mushroom.

Comic Books

 * Used to disgusting effect in Warren Ellis' Supergod

Literature

 * The Caterpillar's mushroom in Alice in Wonderland. Eating one side of it makes you grow taller, eating the other side makes you grow shorter.
 * In the Necroscope books mushrooms growing on a vampire's grave can cause someone to be vampirised themselves.
 * An apprentice shaman in The Light Fantastic is drugged up on both sacred toadstools and mystic mushrooms, in the hope of a vision of Topaxci, God of the Red Mushroom. Instead he gets glared at by the Luggage, and would have run away if he'd been capable of such a complex sequence of movements. The same book also has a gnome who lives in a bright red toadstool with white spots, a variety which the aforementioned shaman would only eat after tying himself to a rock.

Tabletop RPG

 * Dungeons & Dragons
 * Two adventures by Gary Gygax himself (who had mushrooms as a Creator Thumbprint)
 * S4 The Lost Caverns of Tsojcanth. One room had mushrooms which gave special benefits if eaten. Red ones cured one Hit Point of damage, purple ones granted infravision and yellow-gray ones gave immunity to poison for a time.
 * EX1 Dungeonland has several types of fungi, including Death Angel (touching caused death), Cup Fungi (touching caused acid damage), Giant Puffball (exploded if struck or punctured), Horsetail Mushroom (if eaten, doubled the rate of recovery of Hit Points and movement speed for 10 minutes) and Toadstool (If touched, turned into a Giant Toad and attacked).
 * Basic D&D adventure O2 Blade of Vengeance had 5 types: Neutralize Poison, Haste, Clairvoyance, Cure Light Wounds (all as the spells of the same name) and nourishing (equals a meal).

Video Games

 * The various versions of mushrooms in Super Mario Bros.. have many different effects. The most famous are the Super Mushroom and the 1-Up Mushroom, but there exists also the Poison Mushroom, the Spring Mushroom, the Sucky Shroom...
 * Cute Witch Kirisame Marisa uses phantasmal mushrooms to power most of her magic according to supplementary material.
 * The Red Mushroom and Blue Mushroom in Boktai respectively make the eater shrink and invisible
 * Various mushrooms can be used with Alchemy in The Elder Scrolls (or eaten raw) for different effects. Played realistically, as the mushrooms also have various negative effects in addition to positive, depending on how you mix the potion.
 * The Legend of Zelda: A Link to the Past had a mushroom that could be taken to a witch to make Magic Powder, which could turn chickens into humans, spirits into fairies and awake sleeping demons.
 * The mushroom and the Magic Powder returned in The Legend of Zelda: Link's Awakening, where it could light fires and kill enemies.
 * And apparently turned a guy into a Tanuki
 * Angband has various types of mushrooms, a few of which have beneficial effects when eaten. Of course, telling which ones are Poison Mushrooms can be tricky without scrolls or spells of identify... And then there's the Mushrooms of Hallucination.
 * Cave Story has the Ma Pignon mushroom, which cures amnesia even when eaten by a robot. In a twist,
 * In the first Quest for Glory game, there is a ring of magic mushrooms somewhere in the forest which is protected by the fairies at night. If they see you go inside their ring, they will dance you to death. Their only true use is to be given to the healer for a gold coin apiece. Eating one will make the screen briefly flash with colors, and eating more will kill you.
 * Metal Gear Solid 3: "These Glowing Mushrooms recharged my batteries!"
 * the Hypno-shroom in Plants vs. Zombies which causes a Mook Face Turn
 * Enough Plumbers, a Super Mario Bros parody, turns the mushroom powerups into psilocybin shrooms.
 * The Pokémon Paras is an insect-like creature with a pair of mushrooms growing on it's back. In it's evolved form Parasect, the Mushrooms have merged into a single cap and taken over the bug's higher brain functions.
 * And Gen V has Foongus and its evolution Amoongus, apparently sentient mushrooms whose spore releases can sicken and poison people or Pokemon in the area.
 * The Adventures of Robin Hood has magic mushrooms that will bring you Back from the Dead.
 * In a reference to Alice in Wonderland, King's Quest has a shrinking mushroom that you need to get out of the Land of the Leprechauns and back to the west side of the river. Wasted it or went underground without it? Congrats, now your game is Unwinnable.
 * Dungeons of Dredmor has a variety of interesting fungi that your character can collect. All can be eaten for various effects, most of which are good (except for the poisonous Mud Wen), and many can be used as ingredients for alchemy. There's even a whole skill tree devoted to mushrooms, Fungal Arts.
 * Hasardous House in Something is based around the gimmick of the ? Mushroom. Its effects are randomized.