In short, this trope references the development of immunity to a particular drug or poison by taking small doses for a long time.
Here's a typical scenario: The hero has finally appeared at his confrontation with the Big Bad, who's seated at his big table, just about to take his evening meal. "There's no reason to be uncivil," the villain says. Would the hero like some wine? The hero takes a drink, and immediately starts choking. The villain laughs - that fool, the hero, should have known that the villain would poison the wine with the dreaded juice of the Killemall fruit!
But what's this? The hero's standing back up! "I knew you'd poison the wine with the dreaded juice of the killemall fruit. That's why I've spent years eating small pieces of killemall fruit, to develop an immunity to the poison!" The hero then kicks the villain's tail.
In some cases, the poison builds up and actually turns the poison-proof character into a Poisonous Person.
This can be Truth in Television, or not, depending on the poison in question. For some poisons, the body produces antibodies to clear them from the system; so, with repeated exposure to small amounts, you can build up a level of circulating antibody that grants immunity to a typical dose. However, there are plenty of other poisons that don't get cleared from the system and simply build up in your tissues until you reach a lethal dose.
The official term for this is Mithridatism, after a king who made use of the effect. It backfired when he was defeated and tried to commit suicide; his immunity to poison worked so well that he ended up needing to hire a mercenary to run him through.
Could be considered a sub-trope of Adaptive Ability. A particularly Crazy-Prepared person may be immune to several — or even all — poisons via this method.
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Killua from Hunter × Hunter is immune to virtually all forms of poison due to his family's Training from Hell, and is seen happily downing five cans of laxative-laced juice before the Hunter Exam starts.
He's also "immune" to electricity, eventually making various forms of lightning punches, lightning bolts, and literal lightning reflexes part of his powers.
In Apothecarius Argentum, Argent was fed a number of poisons at a young age so he could be sold as a food taster/assassin. As a side effect, they also turned his blood into some kind of killer acid, and just touching him is enough to make his love interest, the princess faint.
Assassin Shao Li from Noir kills using poisoned fingernail polish and inflicting a small nick on her victim, and in one scene she kills a man using incense that contains a poison she's built up an immunity to and remarks as he's choking on the ground that to her it just smells pleasant.
Shi Ryuuki in Saiunkoku Monogatari built up a resistance to various poisons mostly thanks to growing up as The Unfavorite at the bottom of a pecking order of six princes and their mothers. Sa Sakujun in the same series built up a similar resistance through bored experimentation, not that it does him a lot of good in the end.
Most of the Gourmet Hunters in Toriko have resistance to various poisons due to incidental or deliberate exposure. Coco is an extreme example, having been exposed to so many toxins that he's able to synthesize them within his body, and on the rare occasion that he's hit with a poison that he ISN'T immune to, he can adjust his immune system within seconds.
Played for laughs in One Piece with Magellan, whose exposure to his own poison leaves him stuck on the toilet for ten hours a day.
Less for laughs and more for awesome after the Time Skip, when Luffy's clash with Magellan gives him immunity to every poison in the world. As if he hadn't taken enough levels in badass already...
In Pokémon, Ash has gotten fried by Pikachu so many times that he's built up a near-total immunity to electric shocks, surviving jolts that were outright exploding whatever they hit in a later episode. Meowth also made use of his Pikachu-induced shockproofing once.
AJ's Sandshrew has Acquired Water Immunity.
Zen of Akagami no Shirayukihime, being a prince and thus having a good chance of being poisoned, spent an extended amount of time becoming immune to many different poisons.
Subverted in Bleach. During Mayuri's fight with Szyael, as his bankai's poisonous gases seep over to the injured Uryu and Renji, Uryu initially believes that unlike Renji, he will be safe from the poison, as he was poisoned before in his previous battle with Mayuri. Uryu then shows symptoms of poisoning, and Mayuri mocks the idea that someone could become immune to his poison, saying that it is constantly adapting itself.
In Dokuhime little girls are purchased for the purpose of bringing them up to be Poison Princesses, and exposed from the time they're in their cradles to poison in order to build up immunity. Many don't make it, the successes are trained to be assassins whose very touch is deadly. They have to keep ingesting poison to function properly.
In Akuma No Riddle, Haru Ichinose has gone through so many surgeries that she has started to become resistant to sleep-inducing drugs.
On the topic of Dr Crane, apparently he's gassed himself so often he's become unable to fear anything...except Batman.
The Joker has built up immunity to his trademark poison to the point that mosquitoes writhe in pain after sucking his tainted blood.
Played for laughs in a crossover with Captain America, where the Joker and the Red Skull discover that their signature poisons are so alike that each is immune to the effects of the other.
Also, the Joker apparently immune to Scarecrow's fear gas, as one comic has them team up before Scarecrow sprays the Joker with his fear gas, which only resulted in the Joker smashing Scarecrow over the head with a chair.
In one instance, the fear gas caused the Joker to laugh uncontrollably.
Since it's the Joker we're talking about, it's less likely that it's a result of acquired immunity and more that he's simply too crazy to be afraid.
It's often established that Joker is immune to many toxins including his own Joker Venom due to years of dedicated substance abuse.
Wolverine has assassin Reiko invoke this trope with blowfish toxin, which Jubilee learns while dodging attacks.
Harley Quinn is immune to Poison Ivy's poisons because of all the, uh... time they spent together.
In the animated series this is Handwaved by having Ivy just give her a vaccine against them.
It's actually a somewhat common martial arts technique in the comics Jademan translated for US release in the 80's and 90's. Indeed, most poison immune characters could actually manipulate their immunity so they could cure someone else's poison by drawing a bit of their own blood and feeding it to them.
In one comic, Superboy deliberately exposes himself to pieces of kryptonite, to build up an immunity to those particular pieces, so he can later use them against Kryptonian criminals with being affected himself.
Nearly every depiction of one of his future incarnations has them demonstrate considerably more tolerance to the effects of Kryptonite, implying that this trope is in effect. Particularly if the story has them paired up with his counterpart from the present day, where Future!Supes will often shrug it off while the Present!Supes is vastly weakened. This depends on the incarnation, however; some continuities justify his resistance with other sources, such as a vastly increased store of solar energy.
In the comic New X-Men, the X-Man Wallflower had this happen to her mother. Her mother was seduced by a mutant with extremely powerful pheromones production and control. He pretty much had her dominated completely. This ended when she was impregnated by the mutant and the result was a daughter, Wallflower. Because she was carrying Wallflower who has the same powers as her dad, Wallflower's mother developed an immunity to both Wallflower and her dad's pheromones. This allowed Wallflower's mom to break the control her dad on her mom.
Get Fuzzy. In the strip for May 18th 2013, Bucky says that he consumed two spiders per day for the last four years to build up an immunity to poison. When Satchel points out that the spiders in their house aren't poisonous, Bucky doesn't feel so good.
The titular badger in The Urthblood Saga, among his other powers, has built up an immunity to all but the strongest poisons from this method. One poor ferret who tried to poison him and take over his army learned this the hard way...
In Woody Allen's Bananas, Gen. Vargas has a servant on hand to taste his meals in case they are poisoned. One meal does turn out to be poisoned, but the General eats it, anyway, claiming that he's been poisoned so many times to have developed immunity.
In Thank You For Smoking, terrorists try to kill the main character by covering him in nicotine patches, which would overwhelm any normal person, and leaving him naked on the lap of the Lincoln Memorial. He survives, and recovers fairly quickly, because he'd been chain smoking for years and had built up a superhuman tolerance to nicotine. Unfortunately (fortunately?) it also means he can never smoke again.
In the 2008 movie Get Smart, 99 sprays Max with knockout gas. Max says that he developed an immunity to it, then passes out while cursing, "Oh, it's the new stuff!"
Pat Morita's character in King Cobra is a snake handler who regularly injects himself with doses of snake venom to develop immunity. He's able to shrug off getting bitten by the giant snake once, but after getting bitten a few more times, he weakens and dies.
In Your Highness, The Wise Wizard, Fabious, and Thadeous smoke herbs together. The Wise Wizard and Fabious get stoned immediately and suffer hallucinations, but Thadeous (who regularly smokes pot) is unaffected.
In the third Riddick movie, Riddick has to get through a narrow pass to get out of the desert into the fertile grasslands beyond. The pass houses a muddy pool with a poisonous scorpion creature that paralyzes its prey. He captures a younger, smaller creature, and extracts the poison. He tests it on a young desert dog first, then injects himself with small doses until he's built up an immunity.
In the short film The Bloody Olive, the female lead explains to a bystander that she was vaccinated against all types of poisons, which rendered the villain's poison dart ineffective.
In Kim Newman's The Hound Of The Durbervilless, in a international meeting of bad guys, Rupert of Hentzau passes around a flask that no one takes, since nobody trusts the others. Even after Rupert openly takes a trick, Colonel Moran thinks to himself that he could have built up an immunity, and mentions Madame Sara (another Victorian villain) who does take a trick, likely has made herself immune to all poisons.
The murderer in the Lord Peter Wimsey novel Strong Poison builds up an immunity to arsenic in this way. This does not work in Real Life...though the reference books Lord Peter reads really do exist, and they really do claim it could work.
A Discworld novel or two mentions a food-taster who has ingested so many poisons that he's not only immune to them but can recognise them by taste (very handy). He can also tarnish silver by breathing on it (not so handy)
..and reputedly eats a toad a day to stay in practice.
The vampires in Carpe Jugulum have also built up a resistance to garlic, sunlight, holy water, vampiric OCD, and holy symbols by this method. It backfires, sort of. When they lose the immunity, they realize they're surrounded by the shapes of holy symbols they wouldn't recognize if they hadn't been shown so many different ones becoming immune in the first place.
Played with in Mort. The first King that Mort sees die asks Death how he was killed. Death explained it was by a crossbow. The king laughed and said "And here I have been making myself immune to all of these poisons. There's no immunity to cold steel, eh?"
In The Count of Monte Cristo, old Monsieur Noirtier survives a murder attempt using poison because he has been taking a medicine that contains the same compound, and has built up a resistance to it. Realizing that his granddaughter and heir Valentine is also a target, he starts giving her small doses of his medicine; this saves her life when the poisoner has a go at her. The poisoner later tries again using a different poison, but by then Valentine's Love Interest Maximilien has called in his friend the Count of Monte Cristo, who saves the day in his own inimitable style.
Poisoning is the de facto assassination method of the Nyissans in the Belgariad. So much so that any government official who lives for very long (case in point: Sadi) has not only long since acquired immunity to some poisons, but is trained to recognize many more, and doses himself with antidotes frequently, just in case.
In the Dashiell HammettContinental Op short story "Fly Paper" (1929) a woman wants to poison her abusive boyfriend, but is afraid he'll be suspicious if she gives him something without drinking it herself. After reading The Count of Monte Cristo she takes small doses of arsenic (extracted from fly paper) to build up an immunity, but instead fatally poisons herself. In discussing the case afterward the detectives reveal that the book is wrong; while some people have a natural resistance to arsenic, it's not possible to build up an immunity through controlled exposure. The poison of choice in The Count of Monte Cristo is in fact Brucine, and is subject to Mithridatism.
The A. E. Housman poem Terence, This Is Stupid Stuff" tells about a king who over the course of his life ate small doses of poison in his food to slowly build an immunity to poisons and thus foil potential assassins. This story is used as an allegory; Housman's poem claims that the purpose of his poetry is to inoculate the reader against the evils of the world by describing them in palatable verse.
Liz Williams' The Poison Master averts this: the Master Ari Ghairen modifies his own body with spider and snake genes to be both resistant and toxic, in an effort to keep up with the cold war in his Guild.
The Dornish in A Song of Ice and Fire are rumoured to put extremely low doses of snake venom in their food along with all the spices, which may be an allusion to this trope. It's supposedly done more for the mild numbing, tingly effect and/or Macho Masochism (a la fugu) than a conscious attempt to build up immunity, though... that is, if it's not just a lie they tell to outsiders.
In Kalki's classic Tamil novel Sivakamiyin Sabadam (Sivakami's Vow), the villainous monk Naga Nandi builds up an immunity to cobra venom. It gets to the point that it runs through his veins instead of blood, and cobras come flocking to him, attracted by the scent.
This is a plot point in Sharyn McCrumb's novel If I'd Killed Him When I Met Him.
Harnrim Starangh, a Red Wizard from Elminster's Daughter. "It had taken two years of retching weakness to build up a resistance to killing doses of staeradder", but being able to use a fast-acting poison freely was worth it, since his most dangerous foes were other wizards whom he couldn't expect to quickly defeat by magic.
In The Hunger GamesPresident Snow tried to build up a resistance to all of the poisons used to kill his opponents, but wasn't always successful, hence the smell of blood.
In 'The Journey of The Catechist" Etjole Ehomba can talk to animals, and a snake puts a very slight poison into his waterskin due to his politeness. He then shrugs off a poisoned dart after having built up an immunity. At which point the dart shooter decides to switch to much more effective magic, and kills Etjole outright.
The Disgaeanovels gives an explanation as to why Laharl survived being poisoned by Etna in the game, the reason was that his crazy aunt Yasurl gave him the same poison when he was little and in her care.
In Best Served Cold by Joe Abercrombie, poison-master Morveer keeps himself resistant to many of his own poisons by regularly consuming them.
In the Gaunts Ghosts novel Traitor General it is mentioned that the Nihtgane partisans have built up immunity to the poisons in the Untill fauna.
Sam of Villains by Necessity has developed one of these to every poison he's likely to encounter in his career as an assassin - except alcohol. He actually chides his Evil Twin for poisoning a knife with a toxin they are both immune to while they fight.
Malus Darkblade develops dermal immunity to poison after years of smearing himself with venomous slime of the huge fearsome lizard he uses as a mount, which he'd been doing so that the beast would allow him near it.
In Agatha Christie's Curtain, Hercule Poirot drugs the murderer using his own sleeping pills, which he has been taking for many years. He uses the same gambit as Westley does in The Princess Bride, poisoning both cups while implying that only one cup is poisoned.
A plot point in Gary Jennings' Raptor.
In the Paradox novel Even the Wingless after almost being poisoned with hekkret, a recreational drug for the Chatcaava but a deadly poison to most Alliance races, Eldritch ambassador Lisinthir starts smoking small amounts of hekkret to build up an immunity. Unfortunately he becomes addicted, and it doesn't provide total immunity against the daily attempts to poison him as he starts vomiting blood.
Phoenix and Ashes: The aftermath of his World War I experiences has Reggie taking lots of sedatives in order to sleep at night (enough that he's getting concerned). Alison isn't aware of this, so when she injects him with opiates to keep him prisoner the dosage isn't nearly enough to do the job.
In the Tales of the Five Hundred Kingdoms book The Snow Queen, the Godmother Aleksia is using her Magic Mirror to gather information on a tyrant that she will need to deal with. She notes that the tyrant visits his Alchemist daily for a concoction made of thirty common poisons, which gives him an immunity to all but truly exotic poisons (and the reason why he doesn't need to employ a tester). For the truly exotic poisons (which are usually slow acting), the alchemist would have the antidote handy.
In an episode of Babylon 5, "Intersections in Real Time", Sheridan is being held prisoner by EarthGov and subjected to interrogation. At one point, the interrogator is eating a sandwich with delight and offers it to Sheridan, pointing out that he's eating it with no ill effects. And he assures Sheridan they have no desire to kill him until after he's been broken. It's only after Sheridan finishes eating that the interrogator mentions that it contained a powerful toxin that the latter has built an immunity to. The toxin doesn't kill Sheridan, but makes him very sick, as intended.
The Cape, a 2010-2011 series, used it when the titular character, learning he was dealing with a poisoner, took it upon himself to work up an immunity to everything the guy was likely to utilize. We didn't get to see if the immunities actually HELD, because the guy just tried to run him through.
The Twilight Zone TOS episode "The Jeopardy Room". A Soviet commissar tricks a defector into drinking wine mixed with a sleep drug by drinking first. He built up an immunity to the drug by repeatedly taking increasing doses over time.
In The Vampire Diaries Katherine has built up an immunity to vervain, she can still get disabled by it if taken by surprise by a large enough dose but she gets over it much faster. Stefan uses this technique to get over his addiction to human blood.
In Stargate SG-1, a single shot from the Zat'nik'tel will typically cause extreme pain and/or unconsciousness in subjects. However it seems that years of exposure to hits from this kind of weapon is enough to build up a tolerance. In later series, we routinely see team-members who've been shot with a Zat suffer only mild discomfort and quickly shrug it off, while those who've never been Zatted before get consistently knocked out.
Wraith stunners seem to have the same effect as well: a glancing hit to a limb sometimes fails to knock out characters like Ronon who have been shot many times before.
In a sketch in Human Giant a contestant practices for a literal gas guzzling contest (as in drinking gallons of petrol and yes it shown to be as deadly as that sounds) by drinking various poisons including paibnt thinner to build up an an immunity. he wins but doesn't have a car.
In one episode of NCIS, the victim turns out to have been a hit man and his murder was one of his intended victims. He used snake venom, and the woman kept snakes and developed immunity, although she didn't specify whether this was her intention or just a side-effect of her preferred pets.
In One Thousand Ways To Die, a man decided to pick up a spider and let it bite him to gain immunity to its venom. Naturally, since he had no idea what he was doing, the bite kills him.
In Dino Attack RPG, Amanda Claw built up immunities to several different poisons and toxins by taking small doses during her years as Silencia Venomosa. This protected her from the poisonous gas in the Mindstorms, Inc. office building. She also hoped this would protect her from the XERRD toxins, but evidently that is one immunity she never acquired; still, she held out longer than most of her fellow Dino Attack agents.
In Paizo's Pathfinder, the Alchemist class slowly gains resistance to poison over the first 9 levels of their career before finally becoming completely immune at level ten, presumably using this method.
In Warhammer Fantasy, ogre butchers (wizards that eats all kinds of dangerous things to cast spells) have the immune to poison rule, so one would assume they have built up a very handy poison immunity.
Spirit Of The Century features an endurance stunt called Developed Immunities that makes a character flat-out immune to "common" poisons and gives +2 or +6 to rolls to resist uncommon ones, depending on whether or not the character has encountered them before. A character who combines this stunt with a sufficiently high Endurance trait has nothing to fear from any save perhaps the most outlandish poisons. (As they run on their own versions of the same system, the exact same stunt can also be found in Starblazer Adventures and Legends of Anglerre. The also related but somewhat less pulp-ish The Dresden Files omits it.)
1st Edition: The Rogues Gallery. The NPC Lassiviren the Dark has taken steadily increasing doses of poison over the years. As a result, few poisons affect him.
Drow of the Underdark. During their training, drow have successively larger doses of drow sleep poison and various spider venoms administered to them. This gives them poison resistance ranging from +4 vs. random ingested poisons to +7 vs. spiders' and their own sleep poison.
Dark Sun boxed set DSE1 Dragon's Crown, book "The Road of Fire". The poisoner Wheelock is immune to all poisons found on Athas because of years of exposure to them.
Members of the Assassin Prestige Class received increasing saving throw bonuses to poison as they went up in level due to their use of and exposure to poisons.
Supplement Creature Collection. The Ubantu tribesmen coat their weapons with poison. They've developed a racial immunity to it due to generations of exposure.
Classic Traveller supplement SORAG: Handbook of Organization and Equipment. During the PC creation process it was possible for a SORAG agent to be assigned to the Medical Division. During the assignment the agent could be given an immunity to Truth Drug by injections of small doses of the drug over an extended period under carefully controlled conditions to build up the body's natural resistance. There was a small chance of the agent's body resisting the treatment, in which case no immunity was gained and the agent's Endurance dropped by 1 point.
Hollow Earth Expedition, supplement Mysteries of the Hollow Earth. Cannibal tribes are known for their skill in creating poisons. Some of them include small doses of poison in their meals to build up an immunity to them.
In Mice and Mystics, the first mouse to be captured with at least one poison wound will not receive poison wounds until the end of the chapter.
Nethack (what else?) features varieties of poisonous meats that have a slight chance of providing permanent poison resistance when consumed. You can similarly gain resistance to heat, cold and electricity by eating certain corpses. You can even get immunity to Disintegrator Rays that way.
A similar Roguelike game, ADOM, gives poison resistance to players who eat corpses of giant spiders.
Subverted in yet another roguelike, Ragnarok. While it's possible to acquire poison immunity in a similar manner (though most venomous animals are still poisonous to eat), the poison of the phantom asp is so potent it has a chance to kill even through supposed "immunity."
In Suikoden II, one country has a sighting ceremony where the knight and the ruler mix drops of their blood in a bowl and sip from it. One character builds up a tolerance until he can make his blood lethal to others.
In the same game, the only reason that Riou can eat his sister's "food" is because he's developed an immunity to it.
In Final Fantasy Crystal Chronicles, you meet the Selkie De Nam, who decides that the best way to deal with the deadly Miasma is to try to build up a resistance to it by drinking water with miasma mixed in. It doesn't end well.
In Lost Souls MUD, once you have any degree of poison resistance, exposure to poison will develop it further.
Acquired poison resistance is one of the perks of the assassin class.
One quest has a werewolf character who claims to have overcome his weakness to wolfsbane herb by posing as a merchant and selling it to werewolf hunters, while secretly exposing himself to small quantities of it.
One World of Warcraft Horde quest has the player fight venomhide ravasaurs (venomous raptors) and get splashed with their toxic blood in order to become immune. This is the first step to getting a venomhide ravasaur mount.
Worgen have a racial ability, Aberration, which reduces the duration of curses and diseases they are afflicted with. Presumably because they're already afflicted with the Worgen curse.
Fallout: New Vegas gives you increased resistance to broken limbs if you've already broken them 50 times. Somehow. It's best not to think about it when the cure for a broken limb is often "sleep it off". Even the game acknowledges this with the reward text "Repeatedly breaking bones has led them to become stronger (somehow)."
In the Dead Money DLC, Dean Domino is resistant to the poisonous cloud of the Sierra Madre due to having been a resident of the place for 200 years. Having him as a companion will grant you a temporary immunity towards the cloud.
In Rune Factory 3, your protagonist has a "Poison" skill that goes up whenever he is poisoned by the enemy, or whenever he succesfully poisons one of them with an attack. One of the benefits of raising it is it makes you harder to be poisoned.
In Dragon Age: Origins Oghren has spent so many years mistreating alcohol, that he no longer suffers any negative effects from whatever he drinks. Taken further in Awakening where during the Joining ceremony, upon drinking the concoction (which contains darkspawn blood among other unpleasant ingredients), which typically renders the new Wardens unconscious, he merely burpsand claims it was "not bad".
In additon Awakening introduces the Vitality skill tree, which provides health bonuses. One of the descriptions mentions the character consuming small amounts of toxic materials to build up resistances.
In the Monster Hunter series, the titular monsters get an increase in poison resistance each time the poison status effect is applied. The same is true for KO, paralysis and traps.
In the final route of Duel Savior Destiny following the end of Mudou and Kaede's Duel Boss Fight the latter collapses after winning the fight due to to poison. She returns in the endgame citing this for why it wasn't an outright double kill.
In Mass Effect 2, failure to realise that without constant supervision, a sedated Shepard won't stay that way for long, ends up foiling the Indoctrinated Alliance agents in The Arrival. One medical report in The Arrival even mentions their frustration that it was necessary to increase every round of sedatives administered because Shepard's system simply grew immune to the previous dose, given only four hours earlier.
Probably justified by the same cybernetics that allow them to survive drinking ryncol.
The Elder Scrolls V: Skyrim: An alchemy perk 'snakeblood' gives you 50% resistance to poison, which follows perks that involve creating poisons and eating ingredients — implying you've had so much passive exposure to the various poisons that they don't work on you any more. Bosmer and Redguards players start out with 50% resistance.
The Adventures of Dr. McNinja: Dan McNinja doesn't need to develop an immunity to poisons. His body separates it out and stores it up so he can squirt it out of his eyes. "Like a toad."
Dead Winter: Black Monday Blues' mother evokes "building a tolerance" right before a pair of bad guys collapses.
Gilgamesh Wulfenbach of Girl Genius has immunity to many many things. Because his father "figures that a ruler should be... hard to kill", what with the people across all of Europe who're upset at killing that Mad Scientist or the process of bombing this town... which extends to his heir. This came in useful in the arc where Tarvek suffered a particularly nasty and rare disease — Gil was able to disregard the risk of infection.
Keychain of Creation: Besides disturbing quantities of alcohol, Ten Winds has reportedly been exposed to toxins, drugs and pharmaceuticals of all kinds and quantities from across Creation over his multi-century lifespan. Stack that on top of his already supernatural Exalted metabolism and you have someone who is very hard to poison.
In Something Positive, Kharisma tries to kill Avagadro with cyanide, which he has built up an immunity to after being poisoned by so many people over so many years. He says that he has grown accustomed to the taste, and now puts it on his cereal.
Spacetrawler: Dmitri believes that a person can become immune to stun guns, and has started shooting himself repeatedly in order to acquire it. Results: he acquires a taste for stun-gun shots. And immunity. In that order.
In Frisky Dingo, Killface tries to poison Phil with a "vitoxin" poison, but finds out Phil built up an immunity to it, coincidentally using small doses to help him lose weight. Unfortunately for Phil he didn't build up an immunity to be accidentally shot in the head by a sniper aiming for someone else.
In Metalocalypse, Pickles is immune to the mind-erasing effects of Totally Awesome Sweet Alabama Liquid Snake, and every other drug as well, as the result of doing "government weed" daily since the age of 6.
Snake Eyes in G.I. Joe: Renegades takes multiple hits from poison darts thanks to a built up immunity. They're still enough to weaken him though.
In one segment of Peabody and Sherman, Mr. Peabody used this trope to help the husband of Lucrezia Borgia.
In Young Justice, Aqualad reveals that he is "largely immune" to the jellyfish toxin that Cheshire uses to coat her darts. Largely doesn't mean completely, though: he was weakened by it, more with each dose. Of course this may be a result of his Atlantean biology, and not an acquired trait.
Dan from Dan Vs. has been hit with tear gas and pepper spray so many times that he doesn't feel their effects anymore. He can even tell the differences.
Nigel Thornberry claims in The Wild Thornberrys that he's developed an immunity to poisonous plants by rubbing their juices all over his body.
Dale Gribble in King of the Hill was unaffected by police tear gas claiming that he kills squirrels with stronger gases. Given that he's an exterminator spraying poisonous gases without wearing a mask and while his health slowly deteriorates (alongside with smoking).
Many famously poisonous animals, such as poison arrow frogs and fugu pufferfish, are not toxic in and of themselves, but rather they develop immunities to poisons in the foods they eat (poisonous insects in the first case and poisonous plankton in the second) which they then concentrate in their bodies as a defense against predators going from Acquired Poison Immunity to Poisonous Person status. These animals raised in captivity and fed a non-toxic diet will be non-toxic themselves (farmed fugu has absolutely zero toxin in it).
As noted above, the official term for this (Mithridatism) comes from King Mithridates VI, a king of Pontus. He feared assassination so badly that he took small doses of poison regularly in order to become immune to the poison's effects. This backfired when the king was eventually conquered. He attempted to commit suicide by poisoning himself only to find that he was immune; depending on the version of the story you hear, he then either fell upon his sword or had an underling run him through. In either case, the poem says it best: "Mithridates, he died old."
This is a bit far-fetched. In reality, poison resistance of this sort is extremely specific, and also lapses quickly if the regimen of repeated doses is not maintained. Granted, Mithridates might not have had a month to let his own immunity lapse...
One theory (in Mayor's Poison King) is that he split his last poison with two daughters, also at risk of be captured and taking to Rome. A possibly expired poison at a low dose for a large man who was likely at least partially immune—probably why he needed cold steel instead.
Supposedly this was a very common practice amongst the upper classes in Ancient Rome. At any rate, it is referenced in the Cambridge Latin textbook series with a similar outcome to Mithridates.
The movie Finding Nemo posits this as the reason clownfish can survive life among sea anemones — Marlin insists on Nemo brushing himself against the tendrils each morning in the vein of a human parent telling a child to brush their teeth thoroughly. Scientific theories vary on whether this is Truth in Television or not.
Some species have natural (innate), rather than acquired, poison resistance. Mongooses, for instance, have antineurotoxic and antihemorrhagic factors in their blood by nature, as do other snake-eating species.
Truth in Television for many snake handlers or bee keepers. It's possible to build an immunity to some types of venom by being near-constantly exposed to small doses of them. In fact, people with strong allergic reactions (such as individuals who would normally go into anaphylactic shock from receiving a single bee sting, or unwittingly eating a product containing peanuts) can build up a resistance in such a way, although doing it safely takes hundreds of injections over the course of several years.
Though they are used to build up immunity to diseases rather than poisons, vaccines operate on this principle as well.
As do antivenins: a large animal, such as a horse, is exposed to a venom, of a snake, say, repeatedly at gradually increasing levels until an immunity is developed. A serum is then drawn from which the antivenin is derived. Professional snake handlers who have similarly built up an immunity can actually donate their blood directly to people who have been poisoned in order to save their lives. Although horses are best known for antivenom production, this was mostly because horses were readily available to the people doing the work. Horses are in fact unusually poison-susceptible for their body mass, and also, serum drawn from them can have complications in a human recipient. Sheep are increasingly the preferred intermediary.
The immune response that leads to this immunity is short-lived. While immunity to a virus will linger a lifetime, because the body can mobilize a response and produce antibodies before the virus takes hold and multiplies, poison is injected all at once. Unless someone is kept "hyperimmune" (has a sufficient stock of antibodies actively circulating in the bloodstream to deal with the injected venom) by regular inoculations of the toxin, immune response won't trigger fast enough to matter. So, a snake handler may stay immune by getting bit regularly, but after a hiatus of any substantial length, they will again be vulnerable to bites. Typically, doses are administered every 21 days to maintain hyperimmunity.
According to Deadliest Warrior, the African warlord Shaka Zulu spat poison into his opponent's eyes during battle. He avoided its effects himself by this method, eating small pieces of the plant it came from for years. This may or may not be true.
While you can't build up an immunity to arsenic, you can build up a tolerance. When American soldiers came to the UK in World War II those stationed in Cornwall often came down with arsenic poisoning from the water that the locals could drink with no problems.
This trope ended up backfiring when a man from Russia attempted to swallow small quantities of toxic mushrooms, arsenic, and cyanide daily to strengthen his body and protect himself from death. He later went into convulsions, slipped into a coma, and died without regaining consciousness. As seen here.
African Honey Badgers. Over their life time, they develop some immunity from the poisonous snakes, scorpions, and bees they regularly prey on. In fact, a male bitten on the cheek by a highly toxic puff adder showed signs of severe pain, but recovered fully within five hours. Watch it here.
It also happens with a lot of drugs we don't think of as poisons. The most widespread example might be caffeine: most coffee-drinking adults and energy drink-drinking teenagers acquire a practical immunity to caffeine (in ordinary doses). A cup of coffee or two is rather effective at preventing the withdrawal symptoms than actually stimulating the nervous system of the drinker (though more serious caffeine abuse is still effective). The same thing happens with alcohol, nicotine and most illegal drugs or prescription painkillers and sleeping pills; they are dangerous because the effective dose rises faster than the lethal dose.
People who had been raised drinking Cola and similar caffeinated juices from the earliest age possible have built such an immunity to caffeine that doses which are not normal by any means and would push an untrained guy to the verge of heart attack - that being a few large mugs of coffee, or a few Red Bulls one after the other - would not even stop them from sleeping just afterwards. As there is no such thing as a free lunch or at least a free coffee, this denies the basic reason for which people do drink them, to stimulate the nervous system and stay awake.
According to some theories, the Aztecs got their red skin tone from the arsenic in their systems obtained by taking it over time to build up immunity.
The Hudson River is one of the most toxic waters in the US due to PCB contamination. This has killed most other fish in the river... save for the Atlantic tomcod. Instead of keeling over like the other fishes, only tomcods with a specific gene mutation survived, leaving the breed adapted to thrive in waters contaminated in PCB.
The human body can build up a tolerance to the anti-allergy medication diphenhydramine (Benadryl) if taken repeatedly over a short period of time, meaning that subsequent doses will be increasingly less effective. Fortunately the effects are short-term.
Longer-acting allergy medications such as loratadine (Claritin), certirizine (Zyrtec), and fexofenadine (Allegra) also become less effective during regular use. Severe allergy sufferers must switch between these medications every two weeks for maximum relief.
One can also develop an increasing immunity to ibuprofen (Advil) and other generic painkillers, which is why chronic migraine sufferers need to switch medicines every few months.
In her autobiography, Venezuelan musician and metaphysican Conny Méndez claimed that, when she was young, one of her uncles decided to make the family immune to cyanide, and convinced them of sparkling tiny quantities of the substance in the food and increasing the doses little by little. The thing ended some years later when an apothecary, alarmed with the huge quantities of cyanide bought by the family, sicced the police on them; by then the kids of the family were ingesting without ill effects enough cyanide to kill a normal adult.
While not quite poison immunity, regularly feeding a child with pollen allergies local honey can severely lessen their reaction to pollen.
A pretty famous example is Rasputin the Mad - he supposedly had been taking minute doses of cyanide over a long period, and when his meal was poisoned, he ate it and walked away. It's probably an urban myth, mainly because you can't build a resistance to cyanide...
The Rough-Skinned Newt has enough tetrodotoxin to kill a room full of adult humans, as a result of a ridiculous evolutionary arms race with Gardner snakes. The evidence suggests that each newt eaten adds more to the snake's overall resistance.