Panacea

The panacea, named for a Greek goddess of healing, is a mythical substance able to cure any illness or poison and even prolong one's life. This basic form of wish-fulfillment, a medicine that instantly fixes everything, is an old idea and takes many forms in various mythologies and subsequent fiction.

The strength of its effect varies. A weak panacea will work on natural diseases but might be useless against certain ailments, especially if they are of a magical or evil nature. It might elongate one's lifespan, but must be taken regularly to do so and it may not work forever. A strong panacea, on the other hand, is proof against anything you can think of and might well bestow endless life with one sip, making its user The Ageless.

It takes many forms as well. Classically it's a liquid, but common substances also include mystical fruits, herbs, flowers, or the body part of some mythical creature. It might even be some kind of jewel.

This is not the same as a Healing Potion, though there is a lot of overlap, especially since both are most often liquids. The distinction is that the Healing Potion is geared towards fixing physical injuries rather than invisible and silent ailments. Nevertheless, many cases of one will work as the other, though this is not always so.

See also Healing Potion and Healing Spring, Immortality Inducer, Philosopher's Stone, and Spice Rack Panacea, an advertising trope.

Anime and Manga

 * The Grand Panacea from Baccano. Its perfect form also grants immunity to aging.
 * Chopper's dream in One Piece is to become a living Panacea, capable of curing any illness.

Comic Books

 * Subverted in the first Squadron Supreme miniseries. Science hero Tom Thumb knows that a "panacea potion" exists in the future one of the team's foes comes from, so when a person important to him gets a fatal illness, he goes there to obtain it. Turns out that in the future, people are so healthy that they only need penicillin and a few vitamin supplements to beat any illness....
 * In Creature Tech, the staff at RTI realize they're dealing with the real Shroud of Turin when they realize the blood on the shroud can heal any wound and raise the dead.

Film

 * Rapunzel's hair in Tangled, and the flower that she got her magic from.

Literature

 * In the first (plotwise) book in The Chronicles of Narnia: The Magicians Nephew, the apples of a particular tree in a special garden have the power to cure disease. The scenario was written to reference the garden of Eden, and the fruit enfolded a Secret Test of Character: after taking one, Diggory is told that had he eaten it for his own gain, he'd have been cursed, but because he wanted it for his terminally sick mother, he was able to bring it back home and cure her.
 * The Athelas weed, aka Kingsfoil, in The Lord of the Rings has unspecified medicinal properties; it may not be good for everything, but it is able to cure several ailments, most notably the Black Breath, a curse brought on by contact with the corruption of Sauron. The process of preparing the remedy suggests that magic is involved. The same plant is used in The Silmarillion.
 * In the Belgariad, Garion makes one of these by accident. When asked to make a rosebush bloom with sorcery for his cousin, he refuses because it'd hurt the plant but compromises by creating a new flower from some twigs. This seems to be the end of the matter until well into the Malloreon, when it turns out to be the "sovereign specific", capable of curing any poison, even one previously believed to have no antidote and resist magic curing. Its further medicinal properties are explored by Polgara but never specified on the page.
 * Similarly, in The Elenium by the same author, The High Queen has been administered a poison believed to have no remedy, and her champion must find an object of power to cure her. As it happens, none will suffice but the Bhelliom, a Cosmic Keystone of practically unlimited power. Although the story initially serves up the Bhelliom as a cure to this specific predicament, it quickly becomes a greater MacGuffin by dint of the desire of the Big Bad to possess it by any means.
 * In Harry Potter, a bezoar is a stone taken from the stomach of a goat and can resolve most, though not all, poisons. Despite the seeming mundanity of the origin, bezoars seem to be quite rare as they are mentioned to be expensive, and the school's potion ingredients store contains only a few.
 * As well, phoenix tears, in addition to mending flesh, are strong enough to be the only cure for basilisk venom.
 * Finally, drinking unicorn blood can sustain one's life no matter how serious the affliction, but the drinker is cursed thereafter because of how evil it is to hurt a unicorn.
 * On Gor the Caste of Physicians have created a pretty good panacea in the "Stabilization Serums," a series of shots which effectively make you immortal and stop aging. You can still die due to injuries of course, and it doesn't work 100% of the time; more like 98%. In book 27 there's a newer version which de-ages you 10 years per treatment. And the Priest-Kings - the Insectoid Aliens Powers That Be of Gor - have perfected them even more: Misk the Priest-King is over 6,000,000 years old. The physicians have also cured almost all diseases except "the Holy Disease" which is believed to be a punishment by the Priest-Kings.
 * Lansip fruits in Tales of Kolmar can cure just about anything, and can even reverse aging. However, they're extremely hard to come by, as lansip only grows on an island that is protected by strong storms - ships can only get through about once every hundred years.
 * Codex Alera has the mushroom called the Blessing of Night, which grows in only one place and can heal injuries, poisoning and.

Mythology and Religion

 * Some versions of the Holy Grail myth had it that the Grail could cure anything, grant immortality, etc. This idea was relished by treasure-seekers for whom a relic of unrivaled holiness just wasn't good enough. The same kind of powers have been attributed to any number of other Christian relics.

Tabletop Games

 * Dungeons and Dragons has the Philosopher's Stone item, which can be broken open and used to make a potion which can heal anything up to and including death.
 * Pathfinder also has this item. In addition the Alchemist class can (at high levels, and if they take the relevent skill) make one.
 * The 3rd Edition supplement Creature Collection: The ewe of the Amalthean Ram gives milk that neutralizes non-magical poisons and diseases in anyone who drinks it.

Video Games

 * General: The panacea is one of the Standard RPG Items, and comes in a few flavours. The weakest and most common is a standard "antidote" item that cures Universal Poison. The rarest is a "cure" item that works against a specific "disease" condition. There is also a "panacea" item in most games that fixes all Standard Status Effects.
 * In Tales of Vesperia Yuri, Karol, and Estelle try to use a panacea bottle to heal the big cherry blossom tree in Halure and, from then on, it becomes a regular item that cures both physical and magical ailments.
 * Panacea is a summonable item in Scribblenauts (along with everything else). In the sequel it heals any sick or diseased creature, and renders an already healthy one invincible.
 * The goal of one subplot in Okamiden is to make a perfect medicine in order to cure a terminally ill girl.
 * In Cave Story, you retrieve a "cure-all" pill from the abandoned hospital in the labyrinth. The full curative abilities of this medicine are unknown, but it heals Curly Brace (who is a robot) from unspecified debilitating injuries.

Web Original

 * SCP Foundation has SCP-500, pills that cure any disease, but there's just about fifty of them and they're impossible to replicate perfectly (though knockoffs can work if you're lucky).
 * Later, SCP-427: the Lovecraftian Locket was developed that has eclipsed it in use.