Can't Live Without You

Maybe you've been put through an Emergency Transformation; maybe a magic spell backfired in an epic fashion; maybe someone decided it would be fun to mess with your life (you know, For Science!); or maybe it's just the way things work in The Verse you're currently a denizen of. Any way it went down, you are now literally physically dependent on something (or someone) else for your continued existence. You get bonus points if your counterpart is also dependent on you, but this often is not the case.

If the link also transmits feelings, pain or injury, this trope overlaps with Synchronization.

For cases where the link isn't vital for either part but simply beneficial, see The Symbiote.

This trope is not about people who can't stand to live apart from each other but are still physically able to do so; for those, head over to either Living Emotional Crutch or If I Can't Have You. See also Soul Jar when it also gives a form of immortality.

Anime & Manga

 * Sailor Moon: if the Maijuku dies, Al and En go down as well. Or as En's dub counterpart explains, "if the Tree dies, ALAN, we die!"
 * in Tsubasa Reservoir Chronicle; the former can't live without the latter, since,
 * Arthur and Shalott in Air Gear; Shalott was almost killed when another character kicked through his chest, damaging all his vital organs, so Arthur's organs were transplanted into Shalott. Now Arthur has no organs, so he's dependent on a literal blood connection between him and Shalott for feeding and waste elimination, and will die in a couple hours if the connection is interrupted.
 * In a Naruto filler arc, the title character finds himself deep within one of Orochimaru's hideouts facing The Dragon, Kabuto Yakushi. Kabuto proceeds to use a Secret Technique of the Fuuma clan (who are mostly loyal to Orochimaru at this point) which connects the hearts of the user, Kabuto and the victim, Naruto, through chakra strings. He then proceeds to remove his own still-beating heart from his chest and squeeze it, causing Naruto's heart to take damage as well.
 * Elraine and Kiri of Double Arts, must always be physically touching (normally holding hands) or Elraine will die of the disease she's infected with but he's immune to and staves off.
 * Charlie in Vassalord is a vampire who is dependent on blood from his master Rayflo because, due to Charlie's religious views, he refuses to drink from humans since he sees it as a cardinal sin. (Drinking from Rayflo is also a sin, but in Charlie's eyes it's the lesser of two evils.)
 * In Katekyo Hitman Reborn, one character,  can live only because.
 * In one chapter of Black Jack, the title doctor surgically attaches a young boy with lungs weakened by heart disease to his mother until he can get an organ donor.
 * Ah! My Goddess has the doublet system, in which random pairs from heaven and hell are linked. If one dies, the other dies. After they are linked, their memories are erased—the purpose being so if you kill someone on the other side, you don't know which of your allies would die (it could be you, natch).
 * In Dragon Ball, if Piccolo were to die, then Kami would as well and vice-versa. Also involves Synchronization.
 * Sequence, by Saenagi Ryou, starts out with the main character, Kanata, accidentally freeing a young imprisoned vampire, and getting his heart ripped out for his trouble. The vampire, who isn't actually a bad person, ties their life forces together to keep Kanata alive.
 * This is the case for Eureka in the Eureka Seven movie ending when she . She stated that she could only exist as long as Renton lives and dreams.
 * In The World God Only Knows, Elsie and Keima share collars/chokers that dictate that if one dies, the other will as well, no matter what they die from.
 * In Princess Resurrection, protagonist Hiro needs infusions of chi/blood from Hime (depending on anime or manga) to continue his existence or he dies within days. Okay, he's technically already dead, but it still works in context.

Comics -- Books

 * In the Marvel Universe villainous duo Hammer and Anvil were linked by an alien device that gave them superpowers but also linked their life force. When one was shot in the head, the other also died.
 * Cloak and Dagger of the same universe depend on each other a little less dramatically—without the countervailing influence of the other's powers, each would go (respectively) terminally depressive or destructively manic from the effect of their own.

Films -- Animation

 * In Tangled, Gothel is dependent on Rapunzel's magical hair to keep her perpetually young; without it, she will rapidly age until death.

Films -- Live-Action

 * In Battle Royale II the students are paired up so that if one of them dies the exploding collar of their partner also goes off.

Literature

 * In Dragonriders of Pern, the dragons are symbiotes who literally can't face life without their rider: a dragon who fails to find a compatible rider upon hatching will die. If they bond to a rider and the rider dies, the psychological trauma causes the dragon to cross the Despair Event Horizon and commit suicide. The only exception is a queen dragon whose rider dies while the dragon is waiting for eggs to hatch—she'll hang around until her kids are born and then commit suicide, but will be a shell of her former self throughout.
 * In Howl's Moving Castle by Diana Wynne Jones (and the film of the book by Hayao Miyazaki), a magical contract binds Howl and Calcifer together. So, as is often referenced when Calcifer talks to Sophie, "If I die, Howl dies too."
 * Inverted in Harry Potter, as the prophecy in the fifth books points out, "Neither can live while the other survives." However, you could say that it is played straight,
 * The Wheel of Time
 * Aes Sedai and their Warders have this type of bond. If a Warder kicks it, the Aes Sedai will be hit with pretty fierce depression, occasionally bordering on suicidal. Warders who lose their Aes Sedai generally go whole hog, becoming Death Seekers against either the person who killed their partner or just whatever's at hand. It's very, very difficult to keep a surviving Warder alive, and it almost always requires extreme measures.
 * The Aiel also seem to have a toned down version.
 * Also, there seems to be some sort of link between Rand and that has had all sorts of weird effects ever since.
 * Inverted in Percy Jackson and The Olympians:
 * Due to plot complications involving the source of his agelessness, the eponymous protagonist of the German pulp series Professor Zamorra has recently (as of this writing) become dependent on his friend Rhett Saris not so much to stay alive as to keep his youth; without him, he would revert to his actual chronological age, which while not actually enough to kill him—the series hasn't run quite that long yet—probably would force him to retire from fighting the various forces of evil. (This is complicated by the fact that Rhett himself might yet end up turning into a fairly major demon himself if one of Hell's long-term plans is ever allowed to reach fruition...and they both know it.)
 * In the Honor Harrington novels, it is common for bonded treecats to suicide after their human partner dies. Since the natural lifespan of a treecat is considerably longer than that of a (pre-prolong) human, the bonding pretty much meant that bonding with a human meant that the treecat would sacrifice several decades of their lifespan.

Live-Action TV

 * Stargate SG-1
 * The Jaffa race in the Stargate Verse can't live for long without having a Goa'uld larva inside their pouch, until a medicine that has the same effect is discovered in one of the later seasons.
 * In one story arc, Daniel and Vala put on bracelets that create a link between them, so that to be any more than a few feet away from each other can cause them extreme physical discomfort and eventually death.
 * Star Trek
 * The same thing happened to Picard and Dr. Crusher on Star Trek: The Next Generation, except it was an implant. It also made them able to read the other's thoughts.
 * The Trill and their symbionts in the Star Trek Verse: they share knowledge, and Trills can live just fine without a symbiote, but if a symbiote implanted in a Trill is removed the process invariably proves to be fatal to the host.
 * Star Trek seems to love this one. The Bynars are a race of cyborgs who always live and work in pairs and can't function alone.
 * Farscape used synchronizing bracelets for a few episodes involving negotiations.

Myths & Religion

 * This trope was intentionally invoked in the now-rare Hindu practice of sati—when a man died, his wife burned herself to death on his funeral pyre.
 * Truth in Television for some cultures - Yanomamo rainforest Indians believe that everybody has a kind of animal doppelgänger called a noreshi, which takes the form of aerial or arboreal animals for men or ground-dwelling animals for women. They say that to kill one's noreshi would cause the person drop dead and vice versa. Many intentionally try to find their rivals' noreshi for this reason, as outright murder is a pretty damn stupid idea in a culture where inter-village alliances are about as sturdy as damp tissue.

Tabletop Games

 * Dungeons & Dragons
 * Some races in D&D have their Life Force linked to an item, plant or place, mostly nature spirits or descendants of some such, with direct inspiration from folklore or mythology. Nereids carry a shawl that contains their soul, and someone seizing it and threatening to destroy it can control the creature. Dryads and Hamadryads are linked to a single tree in the forest. Bamboo and River Spirit Folks, from the Oriental Adventures, are synchronized with a specific bamboo grove or river, respectively. Damaging a linked tree or place will weaken or wound those spirits, and destroying it will kill them.
 * 4th Edition actually has an entire race built on this—the Dvati, a species consisting completely of identical twins, with "souls that burn so brightly one body is not enough to contain them". They don't share damage exactly, but are played as one character (with their HP divided in half between two bodies) and have some limitations on what both can do in one round. If one Dvati twin dies, the other takes unhealable ability damage from the shock and loss of the other half of their soul until either they die too or the other twin is raised/resurrected.

Video Games

 * In BioShock (series) 2 it is revealed that if an Alpha Model Big Daddy (the Player Character in the second game, different from the Big Daddies encountered in the first game) is too far away from his Little Sister, he either dies or lapses into a coma.
 * Dragon Age: Wynne is dependent on a friendly Fade spirit to keep her alive.
 * In the second Zone of the Enders game, the protagonist Dingo is critically injured and linked to Jehuty so he can live, at the expense of not being able to survive outside of Jehuty.
 * Similar but opposite to Pern cycle, the Bond in Drakan series connects the dragon and his rider in such way that if the dragon dies, so does the rider, but not vice-versa (justified by the fact that dragons are just that much more powerful). Arokh, for example, has outlived at least one rider already.
 * In Enslaved: Odyssey to the West, the player character is forcibly bonded to a Damsel in Distress Gadgeteer Genius via a Restraining Bolt that will crush his skull if he disobeys her or she dies, because she can't survive in the killbot-infested wasteland on their own.
 * Prominent in the Nasuverse. In the Tsukihime sequel Kagetsu Tohya, it's revealed that when the magus who created a familiar dies, the familiar will also die. The only exceptions are when it is a very advanced creation . For the same reason (mana sustaining their life) Servants in Fate/stay night will quickly die without a Master. However, this isn't an entirely straight example as the person in question can be replaced... but it is still a replacement in a dependency situation.
 * In Knights of the Old Republic II the player character and his mentor are linked by a Force Bond, gaining each other strengths and weaknesses. This isn't exactly convenient, so getting rid of it is one of the major plot points.
 * In Final Fantasy IV, the death of a summoned monster results in the death of the Summoner. This is how dies early in the game. Once it returns to the realm of summoned monsters, the Summoner can no longer be harmed by its death. Which is likely why Rydia always has her monsters show up for a single attack and then teleport away.
 * In the ending for Tales of Destiny 2, dies along with the final boss Elraine when Kyle destroys the huge lens, as it is the only way to ensure the demise of Elraine, much to the dismay of Kyle. Although in the restarted timeline,
 * Alcatraz in Crysis2 is only kept alive by the Nanosuit, and even then, just barely. At a couple of points in the game the player has to use the suits defribillator system to revive him. Though the suit ends up being less of a life support system and more of a symbiote over the course of the game.
 * Late in Kid Icarus: Uprising Dark Pit discovers that if anything serious happens to Pit he's affected by it too, and makes keeping Pit alive his top priority.

Western Animation

 * Code Lyoko: Although Aelita is materialized in the second season, XANA ensured the kids couldn't Cut the Juice on him by linking Aelita to the Supercomputer. She would lapse into unconsciousness whenever the machine is turned off—and a too long interruption would make her heart stop.
 * After Fry is critically injured in a car crash in Futurama, his head is attached to Amy's body to make sure he survives until his body is healed.
 * Likewise to the above Futurama entry, in a "Treehouse of Horror" episode of The Simpsons, Mr. Burns' head was attached to Homer's body.
 * In one episode of Ben 10, Ben and Kevin are captured and forced into Gladiator Games. They are shackled together by an energy shackle so that if one dies, the other does too.
 * In one episode of The Marvelous Misadventures of Flapjack, K'Nuckles angers off a plague rat who Flapjack befriends. All of Stormalong Harbor contracts the Plague, and Flapjack thinks he is the cause, so he sails away to plague island to live the rest of his life. A boat with K'nuckles and many other citizens show up, and he tells Flapjack that they can't live without him. Flap thinks it's a sentimental statement, but then K'nuckles clarifies that they literally can't live without him. His blood contains the antidote.
 * Wakfu: The whole Sadida people is linked with the Tree of Life, a huge sentient tree at the heart of their kingdom. This prove a lethal weakness when Nox decides to drain the wakfu of the Tree of life, and thus of every Sadida alive.
 * This, along with traits of Synchronization, is the case between Demona and Macbeth in Gargoyles.

Real Life

 * Invoked in a thought experiment often brought up about the abortion debate, where you are asked to be connected to a concert pianist or rocket scientist for nine months because they need your blood supply to live.
 * Biological examples:
 * Mitochondria in eukaryotic cells—that is, cells with a nucleus—and chloroplasts in plants started as independent proto- and cyano- bacteria which survived being enveloped by these larger cells and became symbiotic. They have co-evolved to the point where both mitochondria and chloroplasts are considered organelles, and essential ones at that.
 * Many flowering plant relies heavily on insects in process of breeding.
 * Most hight plants can built hightly beneficial symbiotic link with fungus. However, some plants literally cannot live without one.
 * Lichens are another example of tight symbiotic link. They consist of fungus and a photosynthetic partner.
 * BTW, humans are example as well. Human body is a host for some amount of microflora. Most of it located in digestive system, but skin and most mucous membrane have normal microflora as well. Removing this guests may result in problems with digestion or vulnerabilities to some diseases, which must be considered when doctor decides to give you antibiotic treatment.
 * Legions of parasitic lifeforms cannot survive without a host. Different forms of symbiosis, ranging from straight parasitism to hightly tied true symbiosis, are very common in nature.
 * Certain nastier forms of plasmids have developed addiction systems to keep them safe inside their hosts. Basically, the plasmid produces a toxin that will kill its host, as well as an antidote to that toxin that degrades faster. If the plasmid is removed, the toxin will kill the host once the antidote degrades beyond usefulness.