A delusion is a false idea or belief that is held with strong conviction and subjective certainty and is inconsistent with an individual’s educational, cultural and social background. Delusions are not usually amenable to logic and are often immediately recognizable as false and absurd to others.   
Delusions are most commonly induced under the influence of heavy dosages of hallucinogenic compounds, such as psychedelics, deliriants, and dissociatives. However, they can also occur to a lesser extent under the influence of cannabinoids, stimulant psychosis, and sleep deprivation.
This article focuses primarily on the types of delusion that are commonly induced with hallucinogens and other psychoactive substances. It therefore is not an exhaustive examination of all delusions listed within the Diagnostic and Statistical Manual of Mental Disorders (DSM) that are known to occur to people who suffer from chronic psychological conditions, such as bipolar disorder and schizophrenia.
Substance-induced vs Disease-based Delusions
When associated with psychoactive substance use, delusions tend to occur during the peak of a substance’s effect, though they may also occur as a result of withdrawal from substances such as benzodiazepines. In contrast to disease-based delusions, substance-induced delusions are often ‘broken-out of’ after the duration of the substance’s effect, or when they are provided enough evidence to contradict their delusion. It is rare for a delusion to persist after the effects of a substance have waned.
It is difficult to clearly define the boundary between ‘normal’ beliefs and the delusions that occur while on psychoactive substances. In the context of psychoactive substance use, there is a discernible continuum between beliefs held while sober and the fully delusional beliefs held by those experiencing psychosis. Beliefs that are overvalued yet not delusional are commonly encountered at moderate substance intensity levels. Overvalued beliefs are those which are held with unusual fervency, but are not clearly absurd, such as the paranoid conviction that one’s friends are going to call emergency services imminently. With an increase in substance intensity overvalued beliefs can potentially develop into delusions proper.
A common component to delusions and overvalued beliefs is a markedly increased sense of conviction in one’s beliefs. The intensity of this conviction roughly corresponds to the intensity of the substance’s effect. When conviction is increased, intuitive thinking is likewise increased, resulting in abnormally bold (and often tenuous) ideas and claims about science, metaphysics, politics, art and religion.
Types of delusion
Although substance-induced delusions can occur in myriad forms, they can be often categorized as belonging to particular types, the most common of which are documented and described below:
Grandiose delusions are those where there is an inflated sense of personal accomplishment or status, possibly as a product of effects experienced while under the influence of psychedelics or stimulants, including ego-inflation, mania, geometry and internal hallucinations. These delusions persist for the duration of the substance’s effect, but will often be recognized as absurd upon obtaining sobriety.
Common grandiose themes include:
- Delusion of Transcension: The idea that one has ‘transcended to a higher level of being’ and is destined to solve the multitude of the world’s problems using extra-dimensional knowledge.
- Delusion of Enlightenment: The idea that one has obtained an enlightened or god-like status comparable to The Buddha or Jesus and now comprehends the simple answer to life, the universe and everything. This individual may make claims that they have obtained a permanent enlightened status, eliminated their ego, and shall become historically noteworthy. This delusion is more common amongst users of short-acting ego death inducing hallucinogenic substances, such as DMT, nitrous oxide and salvia.
- Delusion of Special Knowledge: The idea that one has figured out, due to their supreme genius, the mathematical, scientific, and philosophical problems that no one before them could.
- Delusion of Special Ability: The idea that one has obtained an (often supernatural) ability that few others have. This may include abilities such as telepathy, levitation or willing the universe into producing particular outcomes.
- Delusion of Precognizance The idea that one knows or can determine events that are bound to occur in the future. These predicted events can range from the mundane (‘My friend is going to call me.’) to the calamitous (‘Russia is about to invade the United States.’)
While these delusions are most often applied to the individual who is undergoing the effects of the substance, they may also be applied to others, such as believing a close friend is in some way extraordinary. This projection may be influenced by a disparity in psychological states, such as that between those who are sober and those under the influence of a psychoactive substance.
Paranoid Delusions are delusions where there is a tremendous sense of fear and conviction that one is being watched, monitored, spied upon, and/or plotted against despite there being no evidence to substantiate such a claim.
Paranoid delusions are most commonly experienced under the influence of heavy dosages of psychedelic compounds. However, they can also occur during extreme sleep deprivation and stimulant psychosis.
Somatic Delusions are delusions that involve the belief that the body, or part of it, is missing, broken, deformed, diseased or dysfunctional. These beliefs are typically bizarre and are distinguished from tactile hallucination by seeming so real to the individual experiencing them as to cause significant emotional distress.
Examples of somatic delusions include:
- The belief that one’s intestines are decomposing.
- That the tension in one’s throat is the result of the vocal cords becoming knotted.
- That there is a parasitic worm crawling around in one’s stomach.
- That a limb is dead, dying or missing.
Delusions of reference
Delusions of Reference (also referred to as ideas of reference) are one of the most common types of delusion. This delusion typically involves the falsely held belief that an insignificant remark, event, coincidence, or object in the person's environment is either a reaction to the individual or has significant personal meaning relating directly back to their life.   
In psychiatry, delusions of reference form part of the diagnostic criteria for illnesses such as schizophrenia, delusional disorder, bipolar disorder, and schizotypal personality disorder. To a lesser extent, they can also be a symptom of paranoid personality disorder. However, delusions of reference are especially common under the influence of heavy dosages of hallucinogens or during stimulant psychosis.
A list of common examples of this type of delusion are described and documented below:
- Believing that people in a passing car are talking about you.
- Believing that people on television or in other forms of media are talking about or directly to you.
- Believing that headlines and news articles are written specifically for or about you.
- Believing that events (even world events) have been deliberately planned for you, or have special personal significance for you.
- Believing that the lyrics of a song are specifically about or for you.
- Believing that electronic devices are sending you secret and significant messages that only you are capable of understanding.
- Believing that objects, situations, and events are being deliberately organized to convey a special or particular meaning to you.
- Believing that the slightest bodily movement of another person has a significant and deliberate meaning.
- Believing that posts on social media have hidden meanings pertaining to you.
Delusions of death & dying
Delusions of Death & Dying are distressing, falsely-held beliefs that one is about to die, is currently dying, no longer exists, or has already died. This delusion may be a result of a diminished sense of self or identity that usually occurs during states of high-level ego death.
These delusions are most commonly experienced under the influence of heavy doses of psychedelics and to a lesser extent dissociatives.
Delusions of guilt
Delusions of Guilt are caused by unfounded and intense feelings of remorse or guilt that lead the person to conclude that one must have committed some sort of deeply unethical act. The supposed unethical act can range from something relatively mild, such as the belief that the person has cheated on their partner; it can also be something much more serious, such as the belief that they have murdered their friends and family.
Delusions of Reified Fiction
Delusions of reified fiction are the unfounded belief that something fictional, such as the plot of a TV show, film, video game, or book, is an actual real-life event. This delusion may manifest as the perception that the fictional events within media that is currently being consumed are genuinely occurring in one's immediate vicinity, or simply that the events in the media are real and currently occurring somewhere in the world. This delusion seems to be a result of high-level immersion enhancement combined with memory suppression creating a state of mind in which somebody is highly engrossed in media while no longer having a functional long-term memory that can recall the difference between reality and fiction.
Delusional DerealizationDelusional derealization is the unfounded belief that the person is currently inside of a video game, dream, or movie and therefore their current actions will not have any real-life consequences. In extreme situations and depending on the person, this delusion can sometimes result in committing crimes or violent acts. It seems to be a result of intense psychosis, derealization, disinhibition and memory suppression combining to create an altered state of mind in which somebody mistakes reality for a fictional hallucination.
Delusional derealization is most commonly experienced under the influence of heavy dosages of hallucinogens and occasionally during stimulant psychosis.
Delusions of Sobriety
A delusion of sobriety typically involves the falsely held belief that one is perfectly sober despite obvious evidence to the contrary, such as severe cognitive impairment, significant motor control loss, and an inability to fully communicate with others.
Delusions of sobriety are most commonly experienced under the influence of GABAergic compounds such as alcohol and benzodiazepines. However, in many cases, this state of mind can stem not just from the delusional belief of one's own sobriety, but also from a sense of denial that is motivated by a need to protect one's ego, or a genuine inability to identify one's own intoxication.
Delusions of Permanent Unsobriety
A delusion of permanent unsobriety is the belief that has permanently changed the operation of their mind and will remain in an unsober state. This can also manifest as a fear that one has broken their brain through substance abuse, despite there being no convincing evidence to substantiate such a claim.
Shared delusions are when a delusional belief is experienced in a group setting and it begins to spread between individuals who are similarly intoxicated.  For example, if one person makes a verbal statement regarding a delusional belief that they are currently holding while in the presence of other similarly intoxicated people, they may also begin to hold the same delusion. This can result in shared hallucinations and a general reinforcement of the level of conviction in which they are each holding the delusional belief.
Shared delusions are most commonly experienced under the influence of heavy dosages of psychedelic compounds.
Delusional parasitosis, also known as Ekbom's syndrome,   is a form of delusion in which victims acquire a strongly held belief that they are infested with parasites, bugs, and insects, whereas in reality, no such parasites are present. 
Sufferers may injure themselves in attempts to rid themselves of the "parasites." Some are able to induce the condition in others through suggestion, in which case the term "folie à deux" may be applicable.   During this state, nearly any marking upon the skin, small object or particle found on the person or their clothing can be interpreted as evidence for the parasitic infestation. If this delusion is ongoing, sufferers may compulsively gather such "evidence" and then present it to medical professionals when seeking help. 
In the context of psychoactive substances, it is particularly common during stimulant psychosis after chronic usage of cocaine. 
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