Mental Illness is a mental or behavioral pattern which causes strife or anguish to one's life. These include behavioral or chemical dysfunction, abnormal disorders, intense fears, chemical dependency and, natural to comics, exposure to substances which affect a person's thought process and actions. It is a common weakness to characters in comic books and a somewhat poorly used trope. Used properly, Mental Illness can be a crutch a character must lean on in a story to either overcome or be taken by it. Used improperly, Mental Illness can be a constant background of ill appropriated stigma. Identifying mental illness in comic books requires notice of a character's behavior as it is or becomes irrational to the social norms of the comic book. For instance, Hoppy the Marvel Bunny may act in strange ways to our eyes but within the context of his story he is perfectly normal.
- American Crusader: Archibald Masters drank alcohol to deal with the stress that his illness provided.
- Detective Chimp: Detective Chimp often sought treatment or therapy to deal with the problems he previously only solved with alcohol from mystical bars.
- The Knight: A wealthy socialite, Cyril turned to drinking and drug use when his mentor and partner was killed in action.
- Joe Chill: A small time criminal who tried to steal a Pearl Necklace from a woman so he could buy alcohol but ended up killing her and her husband.
- Lionel Luthor: Father of Lex Luthor, Lionel was an alcoholic that took his stresses out on his son.
- Major Disaster: Paul Booker was hung over when he used his powers to the detriment of his team which resulted in the severely injury of one and the death of another.
- Mothman: Years of crime fighting took it's toll on Byron Lewis and he turned to liniments and alcohol to dull the pain of his sustained injuries.
- Phantom Lady: Stormy became an alcoholic later in her life and went to the Heartland for rehabilitation. Unable to cope with her sickness, she hurt herself to try and end her life. She was saved by Doll Man who had Miss America heal her of her toxicity which helped her recovery.
- The Question: Renee turned to drinking to deal with the loss of her relationships and preferred the dullness over the pain. She was eventually saved and trained by Vic Sage who promoted her to his standing before perishing.
- Cyclone: Maxine developed atypical depression after her personality alienated her from her friends and peers.
- Christine Trelane: Christine began to notice delusions coupled with postpartum depression.
- Empathy: A young woman with "maniacal" depression.
- Brainiac: Brainiac I suffers from mysophobia, the fear of germs.
- Bruce Wayne: Developed a case of "chiroptophobia", the fear of bats, as a child but faced his fears and became his alter ego Batman.
- Quiz: Quiz suffers from mysophobia, the fear of germs.
- Scarecrow: Jonathan Crane suffers from corvidophobia, the fear of crows.
- Sun Boy: Dirk Morgna suffers from nyctophobia, the fear of darkness.
- Doctor Light: Kimiyo Hoshi suffers from nyctophobia, the fear of darkness.
- Superman: Val-Zod suffered from agoraphobia, the fear of leaving ones home. Although he came over this to save his loved ones and his world.
- Arsenal: Speedy, later known as Arsenal, suffers from a strong heroin addiction.
- Doctor: The Doctor suffers from a powerful heroin addiction which is coupled by his transcendent powers.
- Flashback: Flashback suffers from addiction to crack cocaine.
- Hourman: Hourman suffers from addictions to the chemical his father invented called Miraclo. Hourman, like his father, uses this chemical to become the hero he is.
- Johnny Quick: Johnny Quick suffers from addiction to Speed-Juice.
- Mirror Master: Mirror Master suffers from a deep cocaine addiction.
- Allan Quatermain: Allan Quatermain suffered from an addiction to opiates but seemingly kicked the substance after his adventures with the League of Extraordinary Gentlemen.
- Signalman: Signalman went dark for many years before being found in the Suicide Slum addicted to unknown drugs.
- Thumbelina: Thumbelina is addicted to the shrinking serum which gives her her powers along with a degree of paranoia.
- Lady Flash: Lady Flash was manipulated by Vandal Savage due to her addiction to Velocity 9.
- Speed Demon: Speed Demon gets his speed from Steroid B-19 which quickly drove him mad and made him increasingly violent.
- Marionette: Marionette suffers from the addiction to Kanium which is coupled with her multiple personalities.
- Thomas Wayne (Earth 2): Thomas Wayne suffers from addiction to Miraclo.
- Two-Face: Two-Face suffers from bipolar disorder and posttraumatic stress disorder. He heavily relies on his scarred coin to make decisions and without it becomes incapacitated.
- Marionette: Marionette suffers from dissociate identity disorder in which her personalities each learn a different ability.
- Riddler: Riddler suffers from a form of Obsessive-Compulsive Disorder (OCD), the inability to complete even basic actions without involving a riddle. He is often caught when he leaves clear riddles for his adversary Batman. He is his own worst enemy and he cannot stop, including when the Joker taunted Edward by forcing him into a joke contest.
- Heatwave: Heatwave suffers from an obsession with fire often called pyromania.
- Cluemaster: Cluemaster, like Riddler, suffers from the inability to conduct normal life without leaving clues.
- Mad Hatter: Mad Hatter is strongly obsessed with hats. Early in her career he could satisfy his need for hats by simply stealing them but later in his life he only gained this satisfaction from relationships with his hats. He also suffers from schizophrenia with an obsession with the novel Alice in Wonderland and tries to find the fictional character Alice in the real world.
- Firefly: Firefly is addicted to fire, an obsession often called pyromania.
- Harley Quinn: Harley Quinn's obsession with the Joker caused her to go from a functional doctor to a dysfunctional, abused criminal.
- Trickster (Flash TV Series): Trickster is a naturally insane criminal who's disconnection with reality involved him creating toys to cause destruction to just about anyone.
- The Ventriloquist: Arnold Wesker suffers from multiple personality disorder, and is thus obsessed with his dummy/dissociated personality Scarface.
- Killer Croc: Waylon Jones is afflicted with regressive atavism, meaning he has inherited some of the traits of ancestral species of the human race, over time the primal reptilian part of his brain gains more control of his psyche and has caused him to become a cannibalistic psychopath.
- Jessica Cruz: Due to her PTSD, Jessica suffers from general and social anxiety. General anxiety causes persistent and excessive anxiety and worry about activities or events. Social anxiety involves high levels of fear and avoidance of social situations. Both types of anxiety involve worry that is out of proportion to the actual circumstance, and is difficult to control.
- Pamela Isley (Prime Earth): Both Ivy and Batgirl have stated that she's affected by Seasonal Affective Disorder, a disorder characterized by one's state of mind being highly correlated with the seasons (spring = manic, summer = euphoric, autumn = apathetic, winter = depressed). 
Exposure-Generated Mental Illness
- Cupid: The cobalt treatment which gave her her enhanced abilities causes her to suffer from insanity which drives her to be loved and to love others intensely.
- Human Flame: Human Flame was an idiot that could hardly accomplish the most simple of tasks. Libra was dissatisfied with him and thus place a helmet on his head which turned him into a Justifier after exposing him to the Anti-Life Equation.
- Libra: Libra was controlled and manipulated by DeSaad by involving Justin closely with the Anti-Life Equation.
- Mary Marvel: Mary Marvel was infected with the Anti-Life Equation which not only drove her mad and violent but made her irrationally sexual.
- Psycho-Pirate: Exposure to an otherworldly multiverse caused Roger Hayden to go from a traditional villain to a modern age psychopath. He also served as a kind of addictive substances as his mask would induce positive emotions in his victims.
- The Acrobat: The Acrobat is affected by a disorder called Moonitis in which the Acrobat becomes violent and evil when exposed to a full moon.
- Twister: Twister was susceptible to the cult called the Church of Blood. The leader of the Church, Brother Blood, took Theresa in and experimented on her giving her illusion powers but corrupting her mind.
- Olympian: Incorrectly called the schizophrenic Superman, the Olympian wears a harness called the Golden Fleece which gives him the shared powers of Greek heroes and gods. However, long term exposure to the Fleece causes Aristides to exhibit the personalities and boastfulness of the Greek heroes he is empowered by.
- Joker: Joker was a simple criminal who was pushed into a vat of strange chemicals which bleached his skin and drove him insane. Right?
- Magenta: Magenta, also known as Frances Kane, was a simple girl with magnetic powers who grew more and more challenged by her powerful polarizing abilities. She eventually went insane and took out her frustrations on Wally West.
- Rose Wilson (New Earth): Rose developed psychosis as a side effect of being drugged by Deathstroke, causing her to stab out her own eye.
Possession-Related Mental Illness
- Sasha Bordeaux: Employed as Bruce Wayne's bodyguard, Sasha Bordeaux was infected with the O.M.A.C. Virus which turned her into a kind of superhuman cyborg with enhanced physical skills.
- Hal Jordan: Hal Jordan was driven to wit's end by the destruction of his city. Due to his emotional vulnerability he was susceptible to being possessed by Parallax.
- Star Sapphire: Carol Ferris became the villainous Star Sapphire after being exposed to the Star Sapphire Gem.
- Wizard: The addled mind of the Wizard was tempted by Johnny Sorrow who possessed him and used him to come back to life.
- Fury: Fury was infected by the demonic Blood Avenger which possessed and transformed her body for use in destructive purposes.
- Eclipso: Jean Loring was a terrible criminal that found a black gem in her cell one day. She took hold of it and became possessed by Eclipso.
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