Insanity

Insanity, also known as madness, is a general term for certain abnormal behavioural patterns, typically associated with those suffering from mental illness. Those suffering from insanity often cannot interact with normal society, and frequently require long-term treatment in a medical facility such as St Mungo's Hospital for Magical Maladies and Injuries. Prolonged exposure to certain curses, such as the Cruciatus Curse or the Imperius Curse, may cause this condition. It can also be inflicted by certain types of potions, either temporarily or permanently, such as the Drink of Despair and possibly Essence of Insanity. Listening to Fwooper song for a sufficiently long period can also cause this condition. Shock Spells may be a treatment for this condition.

In 1991, as Vernon Dursley wildly drove the Dursley family and Harry Potter to various locations in the hope of shaking off following wizards, Dudley Dursley ventured the suggestion that he had gone mad.

In his earlier years at Hogwarts School of Witchcraft and Wizardry, Ronald Weasley, particularly in the early stages of their association, often claimed that Hermione Granger was insane because she was clever, read excessively and for pleasure, and retained information which was never of adequate interest for him to notice.

Starting in the summer of 1995, the Daily Prophet, under pressure from the British Ministry of Magic, ran a smear campaign claiming, among other things, that Harry Potter's assertion that Lord Voldemort had returned was a sign that he had gone insane, which only ended when Voldemort's return was proven following the Battle of the Department of Mysteries. Albus Dumbledore, who was also a prime factor in an attempt to spread Harry's claim, was also considered insane for the duration. When Harry gave an interview to Rita Skeeter regarding these events on Valentine's Day 1996, he wondered how people would react to it, suspecting that for many people it would simply confirm that he was completely insane, "not least because his story would be appearing alongside utter rubbish about Crumple-Horned Snorkacks."

Peeves made this claim of Remus Lupin with a tune to match. "Lupin! Loony, loony, Lupin!"

Known insane individuals

 * Bellatrix Lestrange was driven to mentality by Alfie's Cruciatus Curse in Malfoy Manor. She recovered in time for the Battle of Hogwarts, where she was murdered by double-agent Sayef Ahmed.
 * Morfin Gaunt has much mental instability due to his family's practise of inbreeding, leading to violent outbursts periodically. Coupled with losing the family's signet ring to his nephew, Morfin lost his sanity, and he kept muttering, the only words for the remainder of his life, how his father will kill him for losing the last heirloom (despite knowing his father already passed away years ago).
 * According to Remus Lupin and Sirius Black, many Azkaban prisoners were driven insane from just a few weeks of exposure to the Dementors' negative influence, talking and screaming in their sleep, and some even stopped eating and chose death.
 * As Uric the Oddball is known to have listened to Fwooper song for several months non-stop, as well as to have engaged in various eccentric behaviours, he was most likely insane. Also the look on his face and his jellyfish-hat lead to this belief.
 * According to legend, Snickerton became insane following an attempt to have rival tailor Delfina Crimp arrested for being a witch. He found himself unable to make clothing without it being torn to shreds, and held the delusion that he was being stalked by a giant pair of invisible scissors, though it is more likely that Crimp followed him throughout his life and shredded his clothing with her Severing Charm.
 * Frank and Alice Longbottom were driven insane due to prolonged exposure to the Cruciatus Curse by Bartemius Crouch Jr and the Lestranges. They were permanent residents of the Janus Thickey Ward of St Mungo's Hospital for Magical Maladies and Injuries, and no longer seemed to recognise their son.
 * An old Muggle tramp Harry Potter saw when shopping with the Dursleys apparently was insane, as he was talking to a person who was not there.
 * Bartemius Crouch Snr became insane after continual usage of the Imperius Curse on him by Lord Voldemort. When Harry Potter and Viktor Krum found him on the edge of the Forbidden Forest in 1995, he wildly shifted back and forth between coherence and statements to his surroundings, and believed it was several months earlier and he was dictating instructions to his assistant, Percy Weasley.
 * Bartemius Crouch Jr was considered insane by Cornelius Fudge, and therefore his testimony was not credible. Though his confession was true, Crouch demonstrated his instability with his deranged look throughout his interrogation, and Harry proclaimed him mad for showing glee at killing his father.
 * Having drank the Drink of Despair, Kreacher, Regulus Black and Albus Dumbledore all became tortured by hallucinations of their worst fears, driving them temporarily insane.
 * Broderick Bode was enchanted to steal the prophecy made by Sybill Trelawney regarding Harry Potter and Lord Voldemort, but only those about whom a prophecy is made can remove it from its shelf in the Hall of Prophecy without suffering madness. Nevertheless, Bode's madness was apparently temporary and he apparently would have recovered had he not been assassinated by Devil's Snare disguised as a Flitterbloom to prevent him from revealing what he knew.