"Madness, as you know, is like gravity. All it takes is a little push!"A character has been a bit ... off for a while now. Maybe they just haven't been themselves, maybe they've gone as far as painting a Room Full of Crazy while reciting a Madness Mantra, or maybe they've gone through a Madness Makeover, but it's apparent from their actions that their rational mind is losing its grip and they are sliding inch by inch toward insanity. The end result of this varies, depending on the tone of the series and who the character is. If the series is dark and edgy, they may go on a murderous rampage. On the other hand, if it's light and soft, it may just be Played for Laughs, becoming a source for their hilarious dialogue and wacky plans. A villain is likely to have a Villainous Breakdown resulting in Karmic Death, while a hero will just have a Heroic B.S.O.D. and then get better. Occasionally, a character will be seen holding back their insanity and generally keeping it in check, until that final straw breaks the camel's back and they finally snap, having a massive Freak-Out. And most rarely, the Sanity Slippage is the effect of being Gaslighted by a villain. A Super Trope to Sanity Slippage Song. Compare Go Mad from the Revelation, Freak-Out, Villainous Breakdown, Room Full of Crazy, Madness Mantra, Laughing Mad, and With Great Power Comes Great Insanity. If this is a backstory it's Pre-Insanity Reveal.
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- The Killing Joke is one take on the Sanity Slippage that eventually led to the origin of The Joker. But the Joker admits he's an Unreliable Narrator and could be remembering wrong or making the whole thing up. He attempts something similar to crack Commissioner Gordon. He doesn't succeed.
- Rorschach from Watchmen is clearly mentally unstable even before the event that causes him to slide off of the slippery slope.
- Jackson Georges from Ex Machina, Mayor Hundred's former NSA handler fits this trope. In flashbacks the reader is shown how the cryptic shard that Mitchell found when he got his powers slowly drives Jackson to levels of extreme paranoia. He develops an unfounded contempt for Mitchell, claiming that without Mitchell's appearance and the mystery of the shard, he could have foreseen and prevented 9/11. He becomes obsessed with protecting his family from dirty bombs and the like, buying a Hazmat Suit and making tally marks on the wall to represent god knows what. It's clear he's a step away from the deep end. Darkly subverted when in desperation to save their failing marriage, his wife breaks into his work shed and steals the shard. A few seconds near a TV are enough to drive her completely and utterly insane, leading her to kill her daughter, husband, and dog. She even chops off her own arm with the shard itself. It was an incredibly jarring and brutal twist on the slow burn of paranoia the reader had been witnessing for months.
- J. Jonah Jameson from Spider-Man suffered a nervous breakdown late in Marv Wolfman's run, to the point where he believed Spider-Man was stalking him. However, Wolfman left the book before long, and Roger Stern retconned the whole thing away as mind-control.
- In the sixth volume of Supergirl, Krypton's destruction and his inability to save Argo City and his own family have deteriorated Zor-El's sanity to the point he strikes a bargain with Brainiac of all people and becomes Cyborg Superman. Supergirl wonders what happened to his father after hearing about some of his actions in Supergirl (Rebirth).
- In Krypton No More, super-villain Radion got radioactive powers because of an accident. His sanity has been deteriorating since, and he has become a raving, murderous, megalomaniac lunatic.
- Superman vs. the Amazing Spider-Man During the climax, Superman thinks his ex-friend Lex Luthor was always about to lose it... and he's finally lost it.
Superman: Luthor's slipping — the pressure is beginning to get to him! He's always walked a fine line between genius and insanity — and this time, I think he's taken the fatal fall!
- Azrael: Jean-Paul Valley's stint as Batman during the "Knightquest" portion of the Knightfall storyline was this. After being doused with the Scarecrow's Fear Gas, the hypnotic trance known as The System kicked in, haunting Jean-Paul with images of his father and of Saint Dumas, the "creator" of the order he once was part of. As he's haunted by these apparitions, The System drives him to be more brutal and continuously modify the Bat-Costume, the pinnacle being when he allowed Abbatoir to die, condemning his captive to death, modifying the Bat-Costume to the point where it was less Batman and more Azrael and outright kicking Robin out of the Batcave and working against him, alienating Commissioner Gordon, and Bruce Wayne and deciding that all criminals need to be put out of their misery. It's at that point that Bruce decides enough is enough and sets out to take back the Mantle of the Bat.
- In Sonic the Comic in the events leading up to the Sonic Adventure adaptation, Robotnik begins to lose it, first trying to destroy Mobius, and later gathering the Chaos Emeralds, the Freedom Fighters, and Chaos itself to his retreat so they would all die together. After this fails, he becomes little more than a catatonic, drooling mess in Sonic the Comic – Online!.
- In the same vein, Dr. Eggman in Archie Comics' Sonic the Hedgehog goes into a slow burn Sanity Slippage after Sonic, Tails, Knuckles and Amy are able to rescue the captured citizens of Knothole, set them up in the nanite-built city of New Mobotropolis, and defeat the machine that defeated Sonic earlier. Things get worse when Knuckles-as-Enerjak destroys Eggman's city and sky fleet and, by the time issue 200 rolls around, Sonic delivers one last defeat that causes Eggman to flip out, tear his mustache apart and devolve into a blabbering mess.
- After Optimus Prime dies (the first time, that is) in The Transformers, Megatron takes it poorly, and becomes convinced Optimus is still alive and planning something. His madness eventually winds up causing him to shoot at the Space Bridge, which promptly explodes and apparently kills him. Or not, as it turns out Megatron was just Obfuscating Insanity, and using his 'death' as a cover to return to Cybertron. He just miscalculated when shooting the Space Bridge, resulting in a spot of Laser-Guided Amnesia.
- Happens to Shockwave in the UK comics, when he learns he dies in the future. Determined to prevent this, he finds Megatron and sets him on Galvatron (well... it's complicated). Then the two start working together. Shockwave, who didn't expect this, just snaps, and kills anyone who approaches him.
- Though the titular character of Johnny the Homicidal Maniac starts out by no means sane, he somehow manages to get worse over the course of the story, as his telling of the story gets steadily less believable, he gets talkier, less predictable and increasingly suicidal. It helps that he's being targeted by The Corruption (maybe), lives in a Living Labyrinth, and regularly murders people.
- Issue #5 of Hit-Girl shows Mindy having Hallucinations of Big Daddy, who advises her to go out and slaughter Ralph Genovese and the rest of his men in the horrifying ways that she'd been planning.
- In Convergence: The Atom #1, Pre-Flashpoint Ray Palmer has been hearing a voice in his head that may or may not be Ryan Choi's and has been acting considerably loopy ever since he was trapped in Gotham.
- Red Sonja: Queen of Plagues first shows Dark Annisia as a slave alongside Sonja, then as a high skilled military commander. As the plague's true nature is revealed she goes from Well-Intentioned Extremist down to bloodthirsty and indiscriminate murderer.
- Teddy of Plutona slides from hero-worshipping the capes to wanting to become one to pursuing some pretty icky attempts to fulfill that dream.
- Jack O'Lantern, the Arch-Enemy of Ultimate Spider Woman, commits increasingly violent and destructive crimes as the series progresses. He goes from robbing banks to taking people hostage to going on killing sprees to orchestrating a bloody city-wide Mob War. Even in jail, we see him fantasizing about carving up human heads the way people do Halloween pumpkins. What's unusual is that Jack always had these sadistic urges-the Sanity Slippage comes from the escalating nature of his crimes.
- A Crown of Stars: Jinnai, this story's Big Bad, became much more paranoid (and a big time megalomaniac) since he overthrew former Big Bad Winthrop because he realizes that the same thing could happen again; only this time with his head on the chopping black.
- In the Undertale fanfic Alphys Hoek, this gradually happens to Alphys over the course of each chapter, partially as a result of having to deal with the game's fanbase 24/7 and also partially as a result of the unspeakably humiliating things that Mettaton forces her to do on public TV where everyone in America can see her; by the end of the story, she's a literal genderbent Ren Höek who's completely hell-bent on twisting Mettaton's neck in his sleep.
- Axis Powers Hetalia: Austria/Roderich Edelstein in the 1983: Doomsday Stories is portrayed for the most part as undergoing through this. His obsession in finding out whether Hungary survived Doomsday grows increasingly desperate (and irrepressible) over time to the point that others eventually start wondering if he's gone delusional from grief. He doesn't. It's also mentioned to have happened as well with Romano (hijacked by The Mafia as his brother's successor) and Poland (having his personality shattered).
- Thousand Shinji: As the story progresses, the corrupting influence of the Chaos Gods upon Shinji, Asuka and Rei increases, and they became more psychotic and unstable. Shinji and Asuka become increasingly violent, the latter descending into berserker madness during two months Shinji was missing, killing criminals every night to lessen her pain and loss, and Rei turns their apartment into a garbage-filled shrine to decay.
- Calvin goes a little crazy trying to watch a pug in Calvin and Hobbes: The Series.
- Jack in Part 1 of Cave Story Versus IM Meen and for good reason. After being trapped inside an insane pedophile's sick mockery of a yuri pairing fanfic between his girlfriend and his girlfriend's best friend, being forced to work a part-time shift at a restaurant called Diarrhea Bell, being forced to watch the two girls 2-Girls-1-Cup each other right in front of his eyes, and being tickle-tortured and thrown in jail, Jack lets loose with this underrated gem of a scene, cementing this story's status as one of the secretly best fanfictions of all time:
"I am Jack. Woe is me for I am wrongfully imprisoned. I am Jack! Sad and unhappy am I without friends. BOO HOO!! BOO HOO HOO HOO!!! (blows snot into handkerchief and smears it all over the floor)""Emptiness wells up in my tortured soul. I'm going to...GOING TO...(vomits all over the floor)""I am Sue; I also am indeed BLEAUGH! (dumps mixture of random condiments and gross stuff on the floor)""I'm just Toroko but I'm gonna barf too, and the MUSTARD, MUSTARD BARF MUSTARD BARF!!! (dumps ketchup and mustard all over the floor)""(soaks hands with the combined mixture) BEHOLD the GOOEY SLOP of our SORROW! (smears it all over his face) IT IS...SLOBBER...DAMMERÜNG! THE TWILIGHT OF THE CONDIMENTS!!! (removes hands from face and screams to the heavens, palms facing skyward) WEEP FOR THEM, WEEP FOR THEIR SORROW-HO-HOHO-HOHH!!!"
- In the vein of the Cutie Mark Failure Insanity Syndrome described above in the Western Animation section, The Audience by Reality Check states that Trixie herself is diving into one as her special talent isn't magic like most unicorns, but rather stage magic, illusions and theatrical personalities. The constant heckling, jeers and challenges towards her achievements that she was receiving were making her the Jerk Ass that she was portrayed as in canon.
- Total Drama Stranded: Briony gets a major one very quickly when Calvin begins relentlessly bullying her.
- From the sister story, Total Drama Switcheroo, Missy gets an even bigger one.
- Queen of All Oni: Her encounter with Lung leaves Jade with some severe PTSD, which only worsens as time goes on. It finally culminates with her confrontation with Tohru in the Vault of Endless Night, which convinces her the heroes are all out to kill or seal her, at which point she snaps completely.
- The final story of Calvin and Hobbes: The Series has two separate plotlines: one concerning the return of Thunderstorm and another concerning the Slender Man showing up. When the stories intersect, Thunderstorm starts displaying signs of this.
- Nav starts showing signs of this in Diaries of a Madman, as he becomes far more reckless and depraved over the course of the story.
- Spike goes through one throughout Zenith, which stems from his Undying Loyalty to Twilight.
- Memories has Serenity. When we first see her, she's like Pinkie Pie, a happy, cheerful, harmless, fun-loving, chaotic, Cloudcuckoolander of a prankster with a soft spot for Discord. Then she dies and comes back to life. Cut to the epilogue and sequel and she's giving out Slasher Smiles, maniacal laughter, telling Discord that she wants to overthrow his mother and rule in her place, and finally saying that together they'll kill Celestia.
- In Being Dead Ain't Easy, Seto Kaiba slowly loses his mind as the story progresses for a variety of reasons, including guilt over Joey's death, obsessively trying to find a way to save Joey, lack of adequate food and sleep, KaibaCorp not doing well, worry for his own life and Mokuba's, and synchronization with the dying Joey.
- The Rise of Darth Vulcan: The Alicorn Amulet should be doing this to Ted/Darth Vulcan, and the fact that it doesn't appear to be is worrisome for the heroes. Comments by Word of God, and in-universe by the Princesses regarding Ted's "True Self" being appalled at his actions, suggest that he was suffering from this even before he put on the Amulet, from years of convincingly pretending to be a cruel bully in order to avoid bullying himself. As such, the Amulet can't drive him insane, because he was never quite sane to begin with.
- This happens to some characters in The Infinite Loops due to the strain, frustration, and boredom of living through the same events many many times. Sakura is the poster child for this sort of thing, having gone off the deep end and lost all moral restraint due to having come to believe that nothing she does in the loops matters since the loop will just reset.
- Occurs in the Legend of Korra fanfic Book 5: Legends. Temuji’s plan in the final chapters seems half-thought out at best, and gets worse the longer they go without fulfilling it, culminating in a Villainous Breakdown worthy of Azula's.
- The ABC's of Fallout: Equestria, a collection of short stories, has Irradiate, witch tells the story of a mare who becomes a ghoul and slowly goes feral.
- In Ave Sonata, this is apparently why Sonata Dusk is The Ditz.
- In The Story To End All Stories, the villain has gradually descended into madness to the point that he is unrecognizable.
- Estee's Triptych Continuum has one in Permanent Record, wherein a poor guard is driven to utter and absolute madness over time by the antics of the Cutie Mark Crusaders, eventually culminating in their utter certainty that everything which goes wrong is their fault. To be fair, though... usually it is.
- Le Papillon Rising takes Adrien Agreste from Miraculous Ladybug, and slowly twists him into a psychotic, obsessive, slightly socipathic Yandere through use of the Butterfly Miraculous. It's worth noting that this is an AU where Adrien is the main villain.
- Transformers Animated: Cybertronian Genesis: One of the main focuses of the narrative for both the original story and its sequel is Megatron's slow descent into madness as his obsession with defeating Optimus Prime grows. Eventually, it becomes so bad that Starscream calls him out on it. He only manages to snap out of it when Gigatron gets involved.
- Miranda Lawson under goes this in It Got Loose and Killed All Our Guys: The Project Lazarus Story, brought on by a mixture of being asked to bring someone back from the dead, the Illusive Man randomly asking for her to add modifications to Shepardnote , dealing with her (rapidly dying) co-workers, and the sheer isolation. She eventually declares Dead Shepard her best, and only, friend, and feels betrayed when he comes back to life.
- Reaper was already Ax-Crazy in Overwatch canon. In Break My Heart, Break Your Heart, he gets worse. Way, way worse.
- Songs about going insane are quite common, actually. Many examples at Sanity Slippage Song.
- "Stan" by Eminem shows the story of a fanboy with a seriously screwed-up life and a downward spiraling sanity. The video starts with Stan bleaching his hair just like Eminem's, then Stan starts writing letters to him every so often in his room all plastered with posters of Eminem, then he sends Eminem a really pissed off letter complaining about how he missed Stan and his brother at a gig. When his pregnant girlfriend scolds him for wasting his day on watching Eminem, that's the final straw on the back, and he ends up tearing all his posters, tying his girlfriend in his car's trunk, and driving off a bridge.
- Then Eminem sends back a letter, and starts by apologizing for how long it's taken him to finally respond. Trippin'. Best part: In his response letter, Eminem decides to cite a story he heard on the news about a guy who drove his car off a bridge with his pregnant girlfriend in the trunk, in the hopes that it'll get Stan to calm down a bit. Then Eminem realizes the news story was about Stan. "Damn," indeed.
- The Nine Inch Nails album The Downward Spiral is a Concept Album that's about Exactly What It Says on the Tin: the gradual destruction of an unnamed man, from the beginning, to him attempting suicide.
- The Violent Femmes' "Country Death Song" is pure American Gothic: rural guy goes mad from isolation and boredom, throws his little daughter down a well, then broods over his guilt till...you can probably imagine.
- Heavily implied in Melanie Martinez's "Pity Party", about a lonely girl trying desperately to convince herself that it's okay that she's alone on her birthday. The music video takes it to the next level with Melanie dressed in youthful clothes smashing the elaborately decorated cake, setting the curtains on fire, and smiling at the camera.
- It's my party, and I'll cry if I want to, cry if I want to, cry, cry, cryI'll cry until the candles burn down this placeI'll cry until my pity party's in flames
- The whole album is about the main character, Crybaby, going through experiences making her more insane. "Mad Hatter", the last song of the album, clearly set up that Crybaby is now fully embracing her madness.
- Pink Floyd's The Wall chronicles a man slowly alienating himself from the world, causing his mind to collapse on itself until finally what's left of his sanity stages an intervention and forces him to actually deal with his problems.
- The band's previous albums, The Dark Side of the Moon and (especially) Wish You Were Here, were inspired by the real-life sanity slippage of former bandleader Syd Barrett, who had become impossible to work with. "Now there's a look in your eyes / like black holes in the sky". His former bandmates reluctantly dropped him and felt guilty about it ever after. The two albums were enormously successful, but the subsequent non-stop grind of touring to unappreciative audiences caused Roger Waters to suffer a sanity slippage of his own, which directly inspired The Wall. Which was an even more enormous success!
- By Coheed and Cambria's third album, the author in the frame story of their sci-fi epic has started talking to a bicycle and making both death threats and declarations of love to the ex-lover for whom he wrote the story. In the previous albums the author was not a significant character, and the only real sign of his instability was the fact he was writing the kind of material you can make a Prog Rock Concept Album out of.
- Gorillaz: Both Murdoc and 2D show signs of this as of phase 3, rambling and gibbering like never before. 2D seems a bit sharper than earlier chapters, though, while Murdoc's just gotten bolder and more malevolent.
- Napoleon XIV's immortal ''They're Coming To Take Me Away, Ha-Haaa!!" starts off sounding like a love gone wrong, but it turns out he's addressing his dog.
- The Neighbourhood's "Wires" is about an old music friend who is suffering this and the protagonist worrying it could happen to himself eventually.
I can hear it in your voice in your voice, you can't be treated.Mr. Know-it-all has his rein and his fall.At least that's what his brain is telling all.If he said "Help me kill the president" I'd say he needs medicine.Sick of screaming "Let us in".The wires got the best of him.All that he invested in goes straight to Hell.
- Edge began to gradually lose his mind as he kept getting closer to a Hell in a Cell match with The Undertaker. It took Mick Foley to tell Edge that he needed to let loose to have a hope of winning, culminating in a two-word "World of Cardboard" Speech: "I understand."
- The entire summer of 2008 was this for Edge. After losing the World Heavyweight Championship to CM Punk, things fell apart and it was a long, drawn-out ride off the slope for him. The above incident with Foley was just the straw that broke the camel's back.
- Randy Savage became increasingly paranoid of other wrestlers trying to make moves on Miss Elizabeth as time went on, which in truth, they were. But as it is said, just because they're out to get you doesn't mean you aren't paranoid.
- Randy Orton infamously started to lose it after his first world title run, which involved his friend and mentor, Triple H, ousting him (forcefully) from Evolution in a fit of jealousy. Cracks began to show in The Legend Killer's suave persona over the next few years, and a Wrestlemania feud with (and loss to) The Undertaker shoved him over the deep end.
- Brian Pillman, due to an increasing string of injuries cutting down on his in-ring time.
- This is how ECW explained the transformation of narcissistic Johnny Polo to the chronically depressed, manipulative cult figure Raven.
- Having his mask stapled to his head and then being hung by Ring of Honor's Hang Men 3 really took a toll on Delirious's sanity. As the name implies, he was already a Cloudcuckoolander but the experience drove him Ax-Crazy enough to concoct a poison designed to restrict the airway, which he stored in his mouth!
- Mickie James's obsession regarding Trish Stratus started about as innocently as an obsession could conceivably be (she used Trish's moves in new ways, hung around, sung her praises, ect). But then Mickie started giving up her own good fortunes for Trish, first admirably but later...to the detriment of her own health. Then Mickie started dressing like Trish. Then she got jealous of anyone who spent any significant time Trish, be it Trish's preexisting protege Ashley or Trish's boyfriend Chad. When Mickie basically rested her head on Trish's crotch and refused to move it that was enough for Trish but the descent of Mickie's sanity had only just begun.
- R-Truth turned heel after losing out on a WWE Championship opportunity. And after that, his mind.
- Daniel Bryan. Yes! Yes! Yes! Yes! Yes! And after AJ Lee left him at the altar, it turned into: No! No! No! No! No!
- In 2010, poor Shawn Michaels started losing his mind after not being able to beat The Undertaker at WrestleMania XXV. And after WrestleMania XXVI, his career. He regained his sanity at this point.
- In her route to the SHIMMER singles championship belt, Madison Eagles went from a silly lady in pink to obsessively dedicated to prying the belt away from MsChif. After she did, it only got worse, to the point she defined her very existence by the title and became jealously paranoid of it, equating any criticism that might in anyway relate to her, any kind of success by anyone else, as being part of some agenda to take it away...leading to the Memetic Mutation of the SHIMMER belt being an artifact of doom only "demons" like the aforementioned Chif could handle.
- AJ Lee's descent into insanity began when Bryan first dumped her after Wrestlemania — needless to say, he would regret it. It's been over a year and AJ still hasn't regained her sanity yet.
- Su Yung had been slipping toward insanity since at least 2009 thanks to an inferiority complex regarding Tracy Taylor. A tour of Japan was supposed to help her get better but thanks to a few encounters with "Sweet" Saraya in SHINE she seems to have completely lost it in 2014...completely.
- If Su Yung's the long case, Kimberly Maddox is the alarmingly short one, as in two shows in two months short, over the fact she couldn't get a win in the Shine promotion. However, Leva Bates theorized that Kimberly already had narcissistic personality disorder, so she likely didn't start very far up the slope.
- Bleak Expectations: A few days on a dessert note island drive Pip Bin utterly mad, to the extent that he loses track of time, and considers eating himself, only relenting when he realises this is "probably a bit of an own goal". He quickly recovers when he finds someone to boss around.
- Played for Drama with Harry Biscuit's mother, who after her husband died started believing she was literally a biscuit (named Susan), and dunked herself to death. After that, the Biscuit fortune was taken by their rivals, the Flapjacks, and Harry was sent to St. Bastards.
- Subverted with Pip Bin's own mother, who after the death of her husband starts sitting in linen closets and claiming to be a tablecloth married to a curtain, before moving on to bizarre interior decorating tips (replacing the walls with geese, putting scatter cushions on fire), and then believing she's the host of a cooking show. In Georgian England. Only it turns out she was faking it, and the minute her husband returns she goes back to normal.
- A daily occurrence in Warhammer and Warhammer 40,000; living in an endlessly terrible Crapsack World will do that. Witches/psykers are especially vulnerable to Slippage, thanks to the source of their powers being the home of The Legions of Hell.
- Also appears in Warhammer Fantasy Roleplay, it has a mechanic for going insane and lists quite a nice amount of mental illnesses for which your character can suffer
- Call of Cthulhu is famous for introducing the SAN attribute, which drops a little each time the players encounter a monster or cast a spell, or something.
- Everyone in New World of Darkness, from normal humans to blood-hungry vampires to reality-bending mages, has a Karma Meter. As you slide down the slippery slope, you tend to become a little more unstable with each step, until finally, humans are reduced to raving lunatics, vampires and werewolves go nuts and become meat-hungry animals, mages and changelings fly off the deep end and lose the ability to separate reality from fantasy, and prometheans lose hope of ever becoming humans.
- The Old World of Darkness also has sanity-slipping Karma Meters, though not all splats have them.
- The fan-made World of Darkness game Genius: The Transgression is unique in that players start out insane by nature of their profession, and as they lose Obligation start unconsciously altering reality to fit their delusions.
- Exalted loves this trope:
- The Solars gradually experience this as they become more and more godlike, from both the Great Curse and general detachment from humanity and less powerful divine beings.
- Being close to a Primordial War survivor in the First Age can be dangerous, humiliating, and bad for your physical, mental, and social health.
- Also happens to Infernals. They have access to Yozi Charms. Everything the Yozis are, is made of Charms. Most of the Yozis are insane, and their Charmsets tend to be arranged so that the good stuff with no drawbacks is padlocked by something with benefits that makes you a little crazier. The best example of this is Kimbery, who has an entire Charm tree (built on The Power of Hate) locked behind something that permanently skews the sanity of its user to be either more naive or more vindictive, depending on the chosen variant.
- In Fading Suns, Psychics and Theurgists are prone to "Urge" and "Hubris" respectively.
- Characters in Eclipse Phase are very hard to kill permanently due to cortical stacks and backups, fortunately for GMs there's a mechanic called "stress points" that can cause psychological disorders or permanent catatonia if the PC lets them accumulate.
- Any Killer Game Master worth the title can put this into any game.
- Blanche in A Streetcar Named Desire as well as the film adaption with Vivien Leigh.
- Sweeney Todd: The Demon Barber of Fleet Street does this with its eponymous character via "Epiphany".
- The title characters in William Shakespeare's Macbeth and Hamlet gets an obsessive variety.
- The title character of The Phantom of the Opera, though your mileage may vary on how sane he was to begin with. But he's definitely completely off the beam by the end, determined to have Christine at all costs, even if he has to blackmail her into marriage by threatening to kill her lover.
- In Shrek: The Musical, Fiona has a moment of this in "I Know It's Today" from waiting to be rescued from a small room in a tower for over twenty years.
- In The Medium by Gian-Carlo Menotti, Phony Psychic Baba suffers increasingly from delusions of someone touching her throat and childlike voices calling out to her as in one of the séances she and Monica put on. She suspects Toby of being the one who touched her, and interrogates him with increasing violence (being a Cute Mute, Toby never answers).
- Burr shows shades of this in the song "The World Was Wide Enough" during Hamilton. It's particularly evident in the line "This man will not make an orphan of my daughter!"
- Sakura in Fate/stay night. It's like everything anyone ever says to her is another stab at her self confidence. And then Shinji tries to rape her one more time, and then he'll tell Shirou about it. Yeahhhh things kind of go downhill from there. Oh, and she was already eating people in her sleep, passing out frequently and also quietly going crazy anyway. She does get better, however.
- Nine Hours, Nine Persons, Nine Doors has Clover, waking up and being forced into playing the Nonary Game. For a second time. She is fine over this. What gives her a dive in the deep end, is Snake's death. Apparent in the "Axe" ending.
- The author has suggested that something traumatic changed Akane in the previous Nonary Game, leaving her cold, vengeful and capable of eliminating everyone who gets in her way.
- Higurashi: When They Cry has this occur several times, almost once an arc, and often bloodily.
- The sister series, Umineko: When They Cry, reveals in the fifth arc that Natsuhi has slowly been losing it since Kinzo died two years before the story begins.
- Arc 3 of Umineko also has Eva/Eva-Beatrice really losing it, first after finding the gold and then after Hideyoshi is killed. By the end of the arc, she's gone completely apeshit and shoots Battler.
- Jack from Gunnerkrigg Court. He started off as a pretty normal guy, then he briefly got pulled into a Dark World and saw some very unpleasant things. Since then he became increasingly antisocial, paranoid (at one point he starves himself because he's convinced the Court has laced his food with nanobots to track him) and amoral (compare his treatment of the Laser Cows to his treatment of the Guard Robot). Heck, even his appearance became worse over time◊. Fortunately, after Zimmy removed the whitelegs from his head, he's gotten almost back to normal. Oh, and he was right about the food.
- Vaarsuvius spent most of "Don't Split The Party" undergoing this in The Order of the Stick. They ultimately recovered but not before committing an act which may have completely crossed the Moral Event Horizon.
- Jin of Wapsi Square suffers from a bad case of this after destroying the calendar machine. It was actually built at least partially for the purpose of keeping her sane.
There. Good as new, best friend! It's like it never happened. No one can ever blame you for dropping him down the stairs now. Stairs? What stairs! Ha ha ha!
- Karkat starts getting shades of this after a particularly brutal Trauma Conga Line. Two of his friends were murdered in from of his eyes, two others are on killing sprees, he's dragging his newly-blinded friend's body (and accidentally drops him down some stairs, and he gets a message from his girlfriend, pronouncing the details of another murder - and when he tries to contact her, one of the Ax-Crazy trolls responds, hinting that she's been killed.
- Rose starts showing signs of it too, around the time her mother died.
- Garfield Minus Garfield: With no cat to give him sarcastic remarks, Jon Arbuckle slowly descends into madness. Examples include smushing ice cream cones into his face, dressing up for a date... that's in three weeks, jumping in the streets wearing a party hat, chasing cars in the manner of dogs, breaking down sobbing at the most random times, Madness Speak, and, finally, being sent to a Mental Hospital.
- Bittersweet Candy Bowl has Lucy having a mental breakdown when Michael falls off a rock bridge while saving her.
- In Pacificators, one of the best Pacificators they had, Castella Brandsford, snapped and murdered thousands of people before she was killed by a suicidal guy. Ever since, the regular people have hated the people who has powers, because they fear another Castella Brandsford.
- Happens to Benjamin Prester in A Miracle of Science when he believes that his partner Caprice has been killed. He becomes increasingly violent, and he starts acting more like he did in flashbacks - when he had full-fledged Science-Related Memetic Disorder. The two biggest clues are he redoes his hair in a particular style when in Mad Scientist mode, and he introduces himself as Benjamin when fully sane but as Ben when in the throes of SRMD.
- In Distortion Nuzlocke, Johnny goes through this more and more as the story goes on. He was already pretty messed up, but his fantasies cause him even more pain.
- Tekno slips in Sonic the Comic – Online! after her and Shortfuse become trapped in a dimension without modern technology. Tekno being a Teen Genius takes this very poorly and becomes more disheveled by the day. It comes to the point where she's writing manically on the walls and she attempts to murder a villain by beating him to death.