Phlebotinum Dependence

Due to illness, environment, biology or a deliberate design decision (or flaw), a character has a Phlebotinum Dependence -- they must receive regular or continuous doses or exposures to some unusual substance or energy just to maintain a baseline healthy state. If they fail to receive their necessary doses, they are not (or not just) Brought Down to Normal -- the phlebotinum is usually responsible for keeping them up to normal. No, without it, they will quickly weaken and start dying, the latter often slowly and painfully.

This is similar to (and might even be seen as a Sub-Trope of) Phlebotinum Muncher, but the key difference is the level (and severity) of the dependency. In the case of the Muncher, the Phlebotinum is a nutritional requirement, and like any other nutrient, its absence will of course adversely affect the Muncher -- perhaps robbing it of its powers or unusual size -- and like any other deficiency eventually causing it health problems up to and maybe even including death. (And it's often implied that the Phlebotinum is a natural and normal part of the Muncher's diet; it's just unusual or dangerous from our point of view -- the monster version of carbohydrates or vitamin B-12.)

In the case of the character with a Phlebotinum Dependence, though, the phlebotinum is not normally part of his diet or environment, but for whatever reason is now directly and actively responsible for keeping the character alive (and sometimes empowered as well), and its absence will have a far more immediate and drastic effect on the dependent individual. Missing doses will usually case pain, insanity or death -- and sometimes all three, either in sequence or all at once.

To emphasize just how serious matters are, the story will usually provide a few Red Shirts with the same or a similar dependence and then deprive them of the phlebotinum so the audience or reader can see just how agonizingly awful it would be to go without. Heroic characters, however, have Heroic Willpower, so when they are similarly deprived, expect them to keep being awesome right to the end (with much panting, grimacing, and comments of "You Can Barely Stand", to let us know how much pain they're in).

See also Phlebotinum Muncher, Bottled Heroic Resolve, and Withholding the Cure. See also Fantastic Drug, which could be the phlebotinum in question. Compare/contrast Addiction-Powered, which can pair up nicely with this trope, as seen in a couple of the examples below.

Comic Books

 * Superman archnemesis Metallo. Uranium capsules will sustain his robot body for a short time but kryptonite will sustain it indefinitely. Carrying around Superman's weakness in his chest is just icing on the cake.
 * Ultraman, the version from the anti-matter universe that's an evil counterpart to Superman, requires periodic exposure to Anti Kryptonite to maintain his powers. Its unclear whether it simply keeps him powered or keeps him alive. He seems to be in bad shape without it but he's also fighting Martian Manhunter at the time.
 * Parasite is on a strict diet of the life force of living creatures. Superman is a giant buffet for him.
 * In some depictions, Batman nemesis Clayface requires treatments to keep his body from dissolving. Regardless of the situation, he's always looking for something to give him better control of his form.
 * In the recent Green Lantern titles Sodam Yat must now permanently wear a power ring, despite being the bearer of the Ion, to keep him from dying of lead poisoning he received at the hands of Superboy Prime.
 * Likewise, bearers of the red power rings will die if their rings are removed, because the red light replaces their blood with its energy. Only a Blue Lantern can purge it from their bodies.
 * In Daniel Clowes' Captain America parody The Battlin' American, the super serum is extremely addictive, causing heroin-like withdrawal symptoms.

Film

 * Iron Man: Tony Stark needs to keep his electromagnet on so that shrapnel in his chest won't migrate to his heart.  JARVIS even lampshades the irony of the situation.

Literature

 * In the Plague Year Series by Jeff Carlson, a deadly nanotech has covered the planet and exists everywhere below 10,000 ft in elevation. To be able to freely walk below 10,000 ft, someone must have a corresponding "vaccine" nano. Unfortunately, the original nano will never go away, so it and the vaccine nano are now permanent facts of life.
 * In Dune, nearly all humans have been consuming spice for over 10,000 years and have become addicted to it in more ways than one. The Fremen are the most addicted, as spice is present in the very air on Arrakis, which is shown by their "blue-within-blue" eyes.
 * Society itself has become dependent on spice. Even people who don't directly consume it depend on it because spice-imbibing Guild Navigators are the only reliable means of navigating faster than light travel.
 * House Harkonnen also makes a point of poisoning all of the captured Mentat Thufir Hawat's food, then giving him the antidote later to keep him from betraying them.
 * The dinosaurs in Jurassic Park are engineered not to produce lysine, requiring humans to administer it. Pity nobody told the engineers that no vertebrates known to man do it either, and we all still survive. The book and sequels realize this and show how well it worked—i.e. not at all.

Live-Action TV

 * Stargate SG-1 has many examples:
 * A Goa'uld symbiote provides a Jaffa with great health and stamina, as well as regenerative powers, but at puberty the Jaffa become incapable of living without these symbiotes for more than a few hours; it acts as their immune system.
 * Tretonin, a chemical used to remedy the Jaffa's dependence on the Goa'uld for survival.
 * The Ilempiri were incapable of being used as hosts by the Goa'uld, so they were fed a highly addictive drug that only the Goa'uld could manufacture, until the entire species was addicted.
 * The title character of the Sci-Fi Channel's The Invisible Man series needed periodic injections to keep him from going Ax Crazy from the Quicksilver in his system. This flaw is deliberately introduced by the Big Bad to keep his buyers dependent on him..
 * Paul Turner on Strange World was pressured to work as a double agent in exchange for a serum that would keep his aplastic anemia in remission.
 * The Jem'Hadar of Star Trek: Deep Space Nine are addicted to the substance Ketracel White, to keep them loyal to the Founders. It provides all the sustenance they need to survive, removing the need to eat or drink, but they also can't take nourishment from any other source. Depriving a Jem'Hadar of the White causes them to go violently insane and then drop dead. There are a few very rare mutants who don't need the White, but in general even they don't know it.
 * So rare in fact that only one was ever found. And then quickly forgotten about.
 * Two, if you count Taran'atar from the Expanded Universe, specifically chosen for his experience and immunity to the White by Odo to be the Dominion representative on Deep Space Nine.
 * Played with in "Symbiosis", a first season episode of Star Trek: The Next Generation. The Ornarans contracted a plague two hundred years earlier, and believe they need continued "treatments" provided by the Brekkans to stay alive. Upon witnessing two Ornarans receiving a "treatment", Dr. Crusher instantly realizes that the "treatment" is actually a narcotic; the plague is long gone, but the Ornarans find withdrawal so traumatic that they're sure that they're still sick - not to mention that the drugs are making them so stupid they can no longer effectively perform basic maintenance on their ships, let along think clearly about the plague. The Brekkans are fully aware of this, but no longer need to work as long as they can trade the "treatments" for Ornaran goods. While the Prime Directive forbids Picard from revealing the truth to the wronged race, he finds a way to correct the situation by refusing repairs to their few remaining ships. Without the ships, they will have no way to get the "treatments" and will eventually realize they're not actually sick.
 * Of course, the Brekkans are going to have an economic holocaust, but after two centuries of enslaving the other world with needles to the point that they're no longer coherent enough to repair their own ships, you might call it Laser-Guided Karma.
 * In Lexx, Divine Assassin Kai needs protoblood to maintain his undead existence. It stops being an issue after the first season when the crew manages to get a good supply of protoblood.
 * In the 1970s-vintage "invisible man" series Gemini Man, main character Sam Casey was caught in a radiation accident that rendered him invisible -- with the side effect that the invisibility was also slowly killing him. He must wear a watch-like device called a "DNA stabilizer" which suppresses the invisibility and holds his inevitable death at bay; he can still be invisible if he needs to be... but for no more than fifteen minutes a day or his body will suffer more damage from the invisibility than it can cope with, and he will die.

Tabletop Games

 * In the Ravenloft Dungeons & Dragons setting, Ivan Dilisnya uses poison to keep his henchmen loyal, dosing them with a toxin called Borrowed Time. Once exposed, they'll die if he doesn't provide regular doses of this substance, which only he knows how to concoct.

Video Games

 * In the BioShock games, people taking ADAM will go insane if they do it too often... and if they stop after that, there's a good chance it'll kill them. And worse still, Rapture's various businesses used it for almost everything, from sport to cosmetic surgery - up until people started going crazy and attacking other citizens for their ADAM.
 * Templars in the Dragon Age series develop Lyrium addiction over time, officially, because their Anti-Magic powers run on it. Unofficially, it's pretty clear that the Chantry hooks them up on Lyrium on purpose to keep them on a short leash.
 * In Deus Ex: Human Revolution, all augmented humans (with the exception of the main character) must take Neuropozene regularly. Otherwise, their body rejects the augments, and the results are... not fun. This is Truth in Television for many Real Life recipients of organ transplants.
 * The Fallout games are full of a wide assortment of Fantastic Drugs (and mundane ones), any of which can potentially become addictive after only a couple of uses, with the withdrawal symptoms causing stat debuffs whenever you aren't on the drug in question. Luckily, it's pretty straightforward for any doctor to rid you of your addictions (and in Fallout: New Vegas there's a consumable item that does it for you).

Western Animation

 * Cybersix was designed so that she requires "Sustenance" in order to live. With the only source of Sustenance being the man who created her, she is forced to extract it from the creatures that he sends to destroy her.