Acquired Poison Immunity

"Buttercup: And to think...all this time it was your cup that was poisoned. The Man in Black: They were both poisoned. I spent the last few years building up an immunity to iocane powder."

- The Princess Bride

"He gathered all the springs to birth From the many-venomed earth: First a little, thence to more, He sampled all her killing store."

- A. E. Housman

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 they reach a lethal level.

This trope is officially named Mithridatism, after Mithridates VI of Pontus, an ancient 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.

Anime and Manga

 * Killua from Hunter X 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 Ninja Scroll, the character Kagero is a poison taster for her master and has become totally immune to poison. It also means that her bodily fluids are highly toxic, which is good for assassination - not so much for potential love interests.
 * 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 poisons a man using incense that contains a poison she's built up an immunity to.
 * 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.
 * Speaking of Magellan, after he nearly kills Luffy with a cocktail of extremely deadly poisons, Luffy proves resistant to one of those poisons when it's used on him again in a different story arc.
 * 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.

Comic Books

 * Spoofed in The Tick (animation), with a minor character who claims to have been building up an immunity to bullets this way.
 * Also spoofed in the Newspaper Comic Close to Home, where a golfer subjects himself to gradually stronger shocks of electricity to build immunity to lightning strikes.
 * An issue of Batman revealed that the title character regularly devises antidotes and methods of controlled exposure to Scarecrow's fear gas. However, every time Scarecrow attacks Batman with a gas, he changes the formula afterwards so that immunizing against the previous has no effect. Batman does it simply on the off chance that Scarecrow didn't change it this one time. In Batman RIP, Batman reveals that he is immune to many toxins and poisons, and carries antidotes for all the ones he isn't immune to.
 * On the topic of Dr Crane, apparently he's gassed himself so often he's developed an acquired immunity to fear itself.
 * 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 (comics), 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.
 * Chizu in Usagi Yojimbo takes a small dose of poison every day for this reason.
 * 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.

Fan Works

 * Aeon Entelechy Evangelion combines this with Conditioned to Accept Horror in Asuka's EVA pilot training, which included controlled desensitizing to anything that causes sanity loss.
 * 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...

Film

 * The Princess Bride. See Literature.
 * Under Siege 2: Dark Territory has a thug who has been pepper-sprayed so many times, all it does is clear out his sinuses.
 * And then he proves it by using it as a breath freshener.
 * 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 . 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.

Literature

 * The Battle of Wits scene in William Goldman's The Princess Bride. The Man In Black has just tricked Vizzini into consuming poisoned wine, and reveals to Buttercup that the wine he'd consumed was also poisoned; he had developed immunity to the poison via this method.
 * 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).
 * The vampires in Carpe Jugulum have also built up a resistance to garlic, sunlight, holy water, vampiric OCD, and holy symbols by this method..
 * 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. Of course 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 Hammett 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 main character in The Journey of The Catechist gets his water skin poisoned by a snake, in a very low dose, for being nice to it. This gives him immunity to a poisoned dart used by the Big Bad later. Not that it helped much.
 * This has been foreshadowed for the Dornish nobility in A Song of Ice and Fire, as it is rumored that the very best hot sauces that the nobility would be buying contain extremely low doses of snake venom.
 * Never mind the fact that snake venom works best when injected into the victim, not swallowed...
 * 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 First 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 Games.
 * 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 Disgaea novels 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 just about 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.

Live Action TV

 * 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. 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 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.
 * Christopher Walken's Saturday Night Live smooth-talking ladies' man character "The Continental" has been maced so many times he's built up an immunity to it.
 * 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 the Community episode "Introduction to Statistics", Jeff invokes this trope as the reason Annie's crying would no longer work on him. It does not work.
 * In the Psych episode "This Episode Sucks", Lassiter is put to sleep with chloroform and then wakes up saying he's been building up an immunity to chloroform over the years.
 * 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.
 * Law and Order Special Victims Unit episode "Wet". A man is believed to have committed murder by poisonous mushroom spores. He built up an immunity to them through years of exposure.

Tabletop Games

 * 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.
 * In the Pulp RPG Spirit of the Century there is an endurance stunt called Developed Immunities that gives a + 2 to resist any poison the character hasn't previously ingested, and + 6 to any he or she has. This makes the character immune to all but the worst of poisons and unluckiest of rolls.
 * Of course, Dungeons & Dragons has this trope in spades.
 * 2nd Edition
 * 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.
 * 3rd Edition
 * 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.
 * Starblazer Adventures, based on the 1980's British science fiction Comic Book. The Developed Immunities stunt gave immunity to almost all common poisons and strong resistance to uncommon ones, gained through careful exposure to them.

Video Games

 * 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. Heck, 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 2, 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 why The Hero can eat her sister's Poison Mushrooms is this trope.
 * 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..
 * In Lost Souls MUD, once you have any degree of poison resistance, exposure to poison will develop it further.
 * Neverwinter Nights (and D&D 3.5) has acquired poison resistance as part of the assassin class.
 * There's a Pokémon ability that grants this—Immunity. Only Snorlax and Zangoose get it, so one of them is a good ally if the opponent uses Toxic or Toxic Spikes.
 * The next version of Dwarf Fortress is set to have this as a feature.
 * One World of Warcraft Horde quest has the player fight venomhide ravasaurs (basically 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.
 * 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".
 * 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 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 darkspawn blood, which typically renders the new Warden's unconscious, he merely burps.
 * In the Monster Hunter series, the titular monsters get an increase in poison resistance each time the poison status effect is applied. Hopefully this is reset for each new monster, but it prevents a hunter for using poison continuously in long fights. The same is true for KO, paralysis and traps.

Web Comics

 * 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.
 * 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 the Europe who upset at killing that Mad Scientist or bombing this town in process... which extends to his heir..
 * Evidently, the Smoke Knights do it. took a combat drug that would kill someone untrained and then survived an overdose of it (long enough to get medical help). Tarvek's summary of his state after mostly losing a fight with his cousin's chief assassin (they were interrupted) and still pulling daggers out of himself was "poisoned - but I'll get over it".
 * Lifolei of Juathuur... a scary, scary woman.
 * 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.
 * As illustrated here.
 * Spacetrawler: Dmitri believes that a person can become immune to stun guns, and has started shooting himself repeatedly in order to acquire it. Results:
 * 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.

Web Original

 * "Shoot yourself with lower caliber bullets to build up an immunity to larger bullets" Fauxtivational Poster meme.

Western Animation

 * 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.
 * Similar to the SNL example above, Quagmire has also built up an immunity to mace after being pepper-sprayed so many times.
 * In Metalocalypse, Pickles is immune to the mind-erasing effects of Totally Awesome Sweet Alabama Liquid Snake, and pretty much 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.
 * 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.

Truth In Television

 * 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. His subjects often called him the Poison King, and he'd often hold banquets where, just to show off, he'd drink wine he purposely spiked with toxins in front of guests. 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...
 * 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. Scientific theories vary on the subject.
 * 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.
 * It is noteworthy that 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.
 * The natives in San Pedro De Atacama, one of the hottest and driest wastelands in Chile, have historically developed thousands of years worth of immunity to what little water is available. For those who don't know yet, the water is laced with up to sixty times the fatal level of arsenic! However, they are not entirely unaffected, since many develop skin lesions and sometimes cancer from this overexposure.
 * 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.
 * Which is why it's probably best if you never try to imitate something you've only heard about and don't have much experience with.
 * 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.
 * Similarly, this has been found to happen with antibiotics. Recently, doctors have been refusing to prescribe antibiotics to children for basic ear infections, since ear infections tend to get better on their own (and tend to usually be more annoying than dangerous,) and because eventually, over-prescribing antibiotics render them less effective in the event that they catch something more dangerous later on.
 * According to legend, the Aztecs got their red skin tone from the arsenic in their systems obtained by taking it over time to build up immunity.
 * CS gas is regularly used in military training for chemical warfare preparation. Instructors for this training often acquire significant resistance over time.
 * Less biological and more psychological is that it is standard practice for police officers to be hit with a Taser or pepper spray before they carry them. This aids them in fighting through the effects, which is important since pepper spray is effected by wind and the (hard to see) wires of a deployed taser are hazardous.