For women taking psychiatric medication, the decision as to whether continue during pregnancy and whether to take them while breast feeding is difficult in any case; there is no data to guide this decision with respect to preventing postpartum psychosis. There is no data to guide a decision as to whether women at high risk for postpartum psychosis should take antipsychotic medicine to prevent it. For women at risk of postpartum psychosis, informing medical care-givers, and monitoring by a psychiatrist during pregnancy, in the perinatal period, and for a few weeks following delivery, is recommended.

For women with known bipolar disorder, taking medication during pregnancy roughly halves the risk of a severe postpartum episode, as does starting to take medication immediately after the birth.

Folie à deux (; ; French for "madness of two"), or shared psychosis, is a psychiatric syndrome in which symptoms of a delusional belief and sometimes hallucinations are transmitted from one individual to another. The same syndrome shared by more than two people may be called "folie à trois", "folie à quatre", "folie en famille" ("family madness"), or even "folie à plusieurs" ("madness of many").

Recent psychiatric classifications refer to the syndrome as shared psychotic disorder (DSM-IV – 297.3) and induced delusional disorder (F24) in the ICD-10, although the research literature largely uses the original name. This disorder is not in the current DSM (DSM-5). The disorder was first conceptualized in 19th-century French psychiatry by Charles Lasègue and Jean-Pierre Falret and is also known as Lasègue-Falret syndrome.

Bipolar disorder is commonly diagnosed during adolescence or early adulthood, but onset can occur throughout the life cycle. The disorder can be difficult to distinguish from unipolar depression and the average delay in diagnosis is 5–10 years after symptoms begin. Diagnosis of bipolar disorder takes several factors into account and considers the self-reported experiences of the symptomatic individual, abnormal behavior reported by family members, friends or co-workers, observable signs of illness as assessed by a clinician, and often a medical work-up to rule-out medical causes. In diagnosis, caregiver-scored rating scales, specifically the mother, has been found to be more accurate than teacher and youth report in predicting identifying youths with bipolar disorder. Assessment is usually done on an outpatient basis; admission to an inpatient facility is considered if there is a risk to oneself or others. The most widely used criteria for diagnosing bipolar disorder are from the American Psychiatric Association's (APA) "Diagnostic and Statistical Manual of Mental Disorders", Fifth Edition (DSM-5) and the World Health Organization's (WHO) "International Statistical Classification of Diseases and Related Health Problems", 10th Edition (ICD-10). The ICD-10 criteria are used more often in clinical settings outside of the U.S. while the DSM criteria are used clinically within the U.S. and are the prevailing criteria used internationally in research studies. The DSM-5, published in 2013, included further and more accurate specifiers compared to its predecessor, the DSM-IV-TR. Semi structured interviews such as the Kiddie Schedule for Affective Disorders and Schizophrenia (KSADS) and the Structured Clinical Interview for DSM-IV (SCID) are used for diagnostic confirmation of bipolar disorder.

Several rating scales for the screening and evaluation of bipolar disorder exist, including the Bipolar spectrum diagnostic scale, Mood Disorder Questionnaire, the General Behavior Inventory and the Hypomania Checklist. The use of evaluation scales cannot substitute a full clinical interview but they serve to systematize the recollection of symptoms. On the other hand, instruments for screening bipolar disorder tend to have lower sensitivity.

This syndrome is most commonly diagnosed when the two or more individuals concerned live in proximity and may be socially or physically isolated and have little interaction with other people. Various sub-classifications of "folie à deux" have been proposed to describe how the delusional belief comes to be held by more than one person :

- Folie imposée is where a dominant person (known as the 'primary', 'inducer' or 'principal') initially forms a delusional belief during a psychotic episode and imposes it on another person or persons (known as the 'secondary', 'acceptor' or 'associate') with the assumption that the secondary person might not have become deluded if left to his or her own devices. If the parties are admitted to hospital separately, then the delusions in the person with the induced beliefs usually resolve without the need of medication.

- Folie simultanée describes either the situation where two people considered to suffer independently from psychosis influence the content of each other's delusions so they become identical or strikingly similar, or one in which two people "morbidly predisposed" to delusional psychosis mutually trigger symptoms in each other.

Folie à deux and its more populous cousins are in many ways a psychiatric curiosity. The current Diagnostic and Statistical Manual of Mental Disorders states that a person cannot be diagnosed as being delusional if the belief in question is one "ordinarily accepted by other members of the person's culture or subculture" (see entry for delusion). It is not clear at what point a belief considered to be delusional escapes from the "folie à..." diagnostic category and becomes legitimate because of the number of people holding it. When a large number of people may come to believe obviously false and potentially distressing things based purely on hearsay, these beliefs are not considered to be clinical delusions by the psychiatric profession and are labelled instead as mass hysteria.

One important factor may be differences or changes in parts of the brain known to be involved in representing body shape (e.g., see proprioception and body image). A neuroimaging study of two people diagnosed with clinical lycanthropy showed that these areas display unusual activation, suggesting that when people report their bodies are changing shape, they may be genuinely perceiving those feelings.

In most cases hospital admission is necessary. Antipsychotic drugs and mood stabilizing drugs such as lithium are typically administered but is not clear if mood stabilizers can be titrated to a high enough level quickly enough to be effective. Electroconvulsive therapy may be considered, especially if there is a high risk of suicide.

Family support may be provided via a social worker.

Clinical lycanthropy is a rare condition and is largely considered to be an idiosyncratic expression of a psychotic episode caused by another condition such as schizophrenia, bipolar disorder or clinical depression.

However, there are suggestions that certain neurological conditions and cultural influences may result in the expression of the human-animal transformation theme that defines the condition.

Treatment of secondary forms of delusional parasitosis are addressed by treating the primary associated psychological or physical condition. The primary form is treated much as other delusional disorders and schizophrenia. In the past, pimozide was the drug of choice when selecting from the typical antipsychotics. Currently, atypical antipsychotics such as olanzapine or risperidone are used as first line treatment.

However, it is also characteristic that sufferers will reject the diagnosis of delusional parasitosis by medical professionals, and very few are willing to be treated, despite demonstrable efficacy of treatment.

There are several other mental disorders with symptoms similar to those seen in bipolar disorder. These disorders include schizophrenia, major depressive disorder, attention deficit hyperactivity disorder (ADHD), and certain personality disorders, such as borderline personality disorder.

Although there are no biological tests that are diagnostic of bipolar disorder, blood tests and/or imaging may be carried out to exclude medical illnesses with clinical presentations similar to that of bipolar disorder. Neurologic diseases such as such as multiple sclerosis, complex partial seizures, strokes, brain tumors, Wilson disease, traumatic brain injury, Huntington's disease, and complex migraines can mimic features of bipolar disorder. An EEG may be used to exclude neurological disorders such as epilepsy, and a CT scan or MRI of the head may be used to exclude brain lesions. Additionally, disorders of the endocrine system such as hypothyroidism, hyperthyroidism, and Cushing's disease are in the differential as is the connective tissue disease systemic lupus erythematosus. Infectious causes of mania which may appear similar to bipolar mania include herpes encephalitis, HIV, influenza, or neurosyphilis. Certain vitamin deficiencies such as pellagra (niacin deficiency), Vitamin B12 deficiency, folate deficiency, and Wernicke Korsakoff syndrome (thiamine deficiency) can also lead to mania.

A review of current and recent medications and drug use is considered to rule out these causes; common medications that can cause manic symptoms include antidepressants, prednisone, Parkinson's disease medications, thyroid hormone, stimulants (including cocaine and methamphetamine), and certain antibiotics.

Boanthropy is a psychological disorder in which a human believes himself or herself to be a bovine.

In primary delusional parasitosis, the delusions comprise the entire disease entity: there is no additional deterioration of basic mental functioning or idiosyncratic thought processes. The parasitic delusions consist of a single delusional belief regarding some aspect of health. This is also referred to as "monosymptomatic hypochondriacal psychosis", and sometimes as "true" delusional parasitosis. In the DSM-IV, this corresponds with "delusional disorder, somatic type".

A fixed fantasy — also known as a "dysfunctional schema" — is a belief or system of beliefs held by a single individual to be genuine, but that cannot be verified in reality. The term is typically applied to individuals suffering from some type of psychiatric dysregulation, most often a personality disorder.

The term is also used in the different context of psychoanalysis to distinguish between 'a normal transitory one and a fixed fantasy' with respect to the phantasised 'fulfilment in conscious or unconscious thought of the sexualised wish'.

More than two-thirds of people diagnosed with BPD also meet the criteria for another Axis II personality disorder at some point in their lives. (In a 2008 study, the rate was 73.9 percent.) Cluster A disorders, which include paranoid, schizoid, and schizotypal, are the most common, with a prevalence of 50.4 percent in people with BPD.

The second most common is another Cluster B disorder, which includes antisocial, histrionic, and narcissistic. These have an overall prevalence of 49.2 percent in people with BPD, with narcissistic being the most common, at 38.9 percent; antisocial the second most common, at 13.7 percent; and histrionic the least common, at 10.3 percent. The least common are Cluster C disorders, which include avoidant, dependent, and obsessive-compulsive, and have a prevalence of 29.9 percent in people with BPD. The percentages for specific comorbid Axis II disorders can be found in the adjacent table.

There is a significant difference between the number of those who would benefit from treatment and the number of those who are treated. The so-called "treatment gap" is a function of the disinclination of the afflicted to submit for treatment, an underdiagnosing of the disorder by healthcare providers, and the limited availability and access to state-of-the-art treatments. Nonetheless, individuals with BPD accounted for about 20 percent of psychiatric hospitalizations in one survey. The majority of individuals with BPD who are in treatment continue to use outpatient treatment in a sustained manner for several years, but the number using the more restrictive and costly forms of treatment, such as inpatient admission, declines with time.

Experience of services varies. Assessing suicide risk can be a challenge for clinicians, and patients themselves tend to underestimate the lethality of self-injurious behaviors. People with BPD typically have a chronically elevated risk of suicide much above that of the general population and a history of multiple attempts when in crisis. Approximately half the individuals who commit suicide meet criteria for a personality disorder. Borderline personality disorder remains the most commonly associated personality disorder with suicide.

After the death of a patient in 2014 NHS England was criticised by a coroner for the lack of commissioned services to support such patients. Evidence was given that 45% of mentally disordered females had BPD and there was no provision or priority for therapeutic psychological services. There were only 60 specialised inpatient beds in England – all in the North East or London.

According to the St. Louis system for the diagnosis of schizophrenia, tangentiality is significantly associated with a low IQ prior to diagnosis (AU Parnas "et al" 2007).

The term refers simplistically to a thought disorder shown from speech with a lack of observance to the main subject of discourse, such that a person whilst speaking on a topic deviates from the topic. Further definition is of speech that deviates from an answer to a question that is relevant in the first instance but deviates from the relevancy to related subjects not involved in a direct answering of the question. In the context of a conversation or discussion the communication is a response that is ineffective in that the form is inappropriate for adequate understanding. The person's speech seems to indicate that their attention to their own speech has perhaps in some way been overcome during the occurrence of cognition whilst speaking, causing the vocalized content to follow thought that is apparently without reference to the original idea or question; or the person's speech is considered evasive in that the person has decided to provide an answer to a question that is an avoidance of a direct answer.

The most famous sufferer of this disorder is claimed to be Nebuchadnezzar II, who in the Book of Daniel "was driven from men, and did eat grass as oxen". Most probably, the authors were making fun of his aversion to meat or violence.

Carl Jung would subsequently instance 'Nebuchadnezzar...[as] a complete regressive degeneration of a man who has overreached himself'.

According to Persian traditions, the Buyid prince Majd al-Dawla was suffering from an illusion that he is a cow, making the sound of a cow and asking that to be killed so that his flesh could be consumed. He was cured by Avicenna.

A spectrum disorder is a mental disorder that includes a range of linked conditions, sometimes also extending to include singular symptoms and traits. The different elements of a spectrum either have a similar appearance or are thought to be caused by the same underlying mechanism. In either case, a spectrum approach is taken because there appears to be "not a unitary disorder but rather a syndrome composed of subgroups". The spectrum may represent a range of severity, comprising relatively "severe" mental disorders through to relatively "mild and nonclinical deficits".

In some cases, a spectrum approach joins together conditions that were previously considered separately. A notable example of this trend is the autism spectrum, where conditions on this spectrum may now all be referred to as autism spectrum disorders. In other cases, what was treated as a single disorder comes to be seen (or seen once again) as comprising a range of types, a notable example being the bipolar spectrum. A spectrum approach may also expand the type or the severity of issues which are included, which may lessen the gap with other diagnoses or with what is considered "normal". Proponents of this approach argue that it is in line with evidence of gradations in the type or severity of symptoms in the general population, and helps reduce the stigma associated with a diagnosis. Critics, however, argue that it can take attention and resources away from the most serious conditions associated with the most disability, or on the other hand could unduly medicalize problems which are simply challenges people face in life.

In medicine, a prodrome is an early sign or symptom (or set of signs and symptoms), which often indicate the onset of a disease before more diagnostically specific signs and symptoms develop. It is derived from the Greek word "prodromos", meaning "running before". Prodromes may be non-specific symptoms or, in a few instances, may clearly indicate a particular disease, such as the prodromal migraine aura.

For example, fever, malaise, headache and lack of appetite frequently occur in the prodrome of many infective disorders. A prodrome can be the early precursor to an episode of a chronic neurological disorder such as a migraine headache or an epileptic seizure, where prodrome symptoms may include euphoria or other changes in mood, insomnia, abdominal sensations, disorientation, aphasia, or photosensitivity. Such a prodrome occurs on a scale of days to an hour before the episode, where an aura occurs more immediate to it.

Prodromal labour, mistakenly called "false labour," refers to the early signs before labour starts.

Studies of borderline children often uncovered at the base of their self-destructive behaviour patterns ' a "fixed fantasy"...a rigid, nonreflective scenario of self-induced pain'. As part of a psychic defence mechanism, 'the omnipotence betrayed by the "fixed fantasy" underlying self-victimization or other forms of self-defeating behaviour...creates the illusory sense that they are actively producing the abandonment [&] pain', rather than merely suffering it passively - 'arranging deceits..arrang[ing] for blows to fall'. Unfortunately 'in the course of development, these patterns acquire multiple adaptive functions...and serve as a key organizer of their sense of self'.

'In producing movement away from fixed fantasy systems, commonplace statements are often necessary because the more fixed and extensive the fantasy system, the fewer the transitional opportunities offered; there is little conflicting material to ride. Banalities may be the only resource', as anything more complex may be used to feed back into the fantasy system itself.

Munchausen syndrome by proxy is a controversial term. In the World Health Organization's International Statistical Classification of Diseases, 10th Revision (ICD-10), the official diagnosis is factitious disorder (301.51 in ICD-9, F68.12 in ICD-10). Within the United States, factitious disorder imposed on another (FDIA or FDIoA) was officially recognized as a disorder in 2013, while in the United Kingdom, it is known as fabricated or induced illness by carers (FII).

In DSM-5, the diagnostic manual published by the American Psychiatric Association in 2013, this disorder is listed under "301.51 Factitious disorder". This, in turn, encompasses two types:

- "Factitious disorder imposed on self" – (formerly Munchausen syndrome).

- "Factitious disorder imposed on another" – (formerly Munchausen syndrome by proxy); diagnosis assigned to the perpetrator; the victim may be assigned an abuse diagnosis (e.g. child abuse).

Factitious disorder imposed on another, also known as Munchausen syndrome by proxy (MSbP), is a condition where a caregiver or spouse fabricates, exaggerates, or induces mental or physical health problems in those who are in their care, with the primary motive of gaining attention or sympathy from others. The name is derived from the term Munchausen syndrome, a psychiatric factitious disorder wherein those affected feign disease, illness, or psychological trauma to draw attention, sympathy, or reassurance to themselves. However, unlike Munchausen syndrome, in MSbP, the deception involves not themselves, but rather someone under the person's care. MSbP is primarily distinguished from other forms of abuse or neglect by the motives of the perpetrator. Some experts consider it to be an elusive, potentially lethal, and frequently misunderstood form of child abuse or medical neglect. However, others consider the concept to be problematic, since it is based largely on supposition regarding a person's motives, which can be open to radically different interpretations.

Factitious disorder imposed on another has also spawned much heated controversy within the legal and social services communities. In a handful of high-profile cases, mothers who have had several children die from sudden infant death syndrome have been declared to have MSbP. Based on MSbP testimony of an expert witness, they were tried for murder, convicted, and imprisoned for several years. In some cases, that testimony was later impeached, resulting in exoneration of those defendants.

A prodrome for schizophrenia is the period of decreased cognitive functioning that is postulated to correlate with the onset of psychotic symptoms. The concept has been reconsidered as the pathways to emerging psychosis have been investigated since the mid-1990s. One example of the international paradigms aimed at researching the prodrome is the North American Prodrome Longitudinal Study (NAPLS). This study is concerned with brain development, hormones, and neuropsychological functions that may play a role in risk for and prevention of mental illness in young adulthood. The term at risk mental state is sometimes preferred, as a prodromal period cannot be confirmed unless the emergence of the condition has occurred. (Also see early psychosis.)

Prodrome, or psychosis risk syndrome as it is also known, is a proposed syndrome to be used in the "DSM-5" (2013) of psychiatry. It is also defined as "the aura that precedes a psychotic break by up to two or three years." Patients with this condition "still have 'insight' – a pivotal word in psychiatric literature, indicating that a patient can still recognize an altered worldview as a sign of illness, and not a revelation." Prodrome is also sometimes called "attenuated psychotic symptoms syndrome." Chairman of the DSM-IV Task Force, Allen Frances, has criticised the concept of Psychosis Risk Syndrome on the grounds of a high rate of inaccuracy, the potential to stigmatize young people given this label, the lack of any effective treatment, and the risk of children and adolescents being given dangerous antipsychotic medication.

About one third of patients with prodrome are diagnosed with schizophrenia or other psychosis in a few years. In the North American Prodrome Longitudinal Study, researchers found that 35 percent of "patients had a psychotic break within two and a half years of enrolling at a clinic." 65 percent "found that their symptoms passed or plateaued." A psychotic break is made statistically more likely (43% vs. 35%) if the patient abuses certain drugs.

The widely used "DSM" and "ICD" () manuals are generally limited to categorical diagnoses. However, some categories include a range of subtypes which vary from the main diagnosis in clinical presentation or typical severity. Some categories could be considered (not meeting criteria for the full diagnosis) subtypes. In addition, many of the categories include a 'Not Otherwise Specified' subtype, where enough symptoms are present but not in the main recognized pattern; in some categories this is the most common diagnosis.

Spectrum concepts used in research or clinical practice include the following.

MdDS is diagnosed several ways, one being by the symptoms: in particular, the "constant rocking, swaying feeling" and the abatement of this feeling when in motion again and as a matter of exclusion. There are no definitive tests that confirm MdDS, only tests that rule out other conditions. Tests include hearing and balance, and MdDS is generally diagnosed by either a neurologist or an ear nose & throat specialist.