Functional somatic syndromes may occur in 6 to 36% of the population.

Treatment may involve investigation, reassurance and explanation, and possibly specialist treatment such as antidepressants or cognitive behavioral therapy.

Functional neurological symptom disorder can mimic many other conditions. Some alternative diagnoses for FND include:

- Hemiplegic migraine

- Multiple sclerosis

- Motor neurone disease

- Parkinson's

- Autoimmune disorders

- Ehlers–Danlos syndrome

- Stroke

- Vitamin B12 deficiency or pernicious anaemia

- Myasthenia gravis

Functional neurological disorder is a common problem, with estimates suggesting that up to a third of neurology outpatients having functional symptoms. In Scotland, around 5000 new cases of FND are diagnosed annually. Furthermore, non-epileptic seizures account for 1 in 7 referrals to neurologists after an initial seizure, and functional weakness has a similar prevalence to multiple sclerosis.

Fitzpatrick et al. (2007) identified 41 children with CVS. The mean age of the sample was 6 years at the onset of the syndrome, 8 years at first diagnosis, and 13 years at follow-up. As many as 39% of the children had resolution of symptoms immediately or within weeks of the diagnosis. Vomiting had resolved at the time of follow-up in 61% of the sample. Many children, including those in the remitted group, continued to have somatic symptoms such as headaches (in 42%) and abdominal pain (in 37%).

Most children who have this disorder miss on average 24 school days a year. The frequency of episodes is higher for some people during times of excitement. Charitable organizations to support sufferers and their families and to promote knowledge of CVS exist in several countries.

Although in itself neither degenerative nor fatal, the chronic pain of fibromyalgia is pervasive and persistent. Most people with fibromyalgia report that their symptoms do not improve over time. An evaluation of 332 consecutive new people with fibromyalgia found that disease-related factors such as pain and psychological factors such as work status, helplessness, education, and coping ability had an independent and significant relationship to FM symptom severity and function.

Fibromyalgia is estimated to affect 2–8% of the population. Female are affected about twice as often as male based on criteria as of 2014.

Fibromyalgia may not be diagnosed in up to 75% of affected people.

Whether a given medical condition is termed a "functional disorder" depends in part on the state of knowledge. Some diseases, including epilepsy, schizophrenia, and migraine headaches were once considered functional disorders, but are no longer generally classified that way.

The average age at onset is 3–7 years, but CVS has been seen in infants who are as young as 6 days and in adults who are as old as 73 years. Typical delay in diagnosis from onset of symptoms is 2.7 – 3 years. Females show a slight predominance over males; the female-to-male ratio is 57:43.

How common the condition is, is not clear. A prospective study found that 3 in 100,000 five-year-olds are diagnosed with the condition. Two published studies on childhood CVS suggest nearly 2% of school-age children may have CVS. However, diagnosis is problematic and, as knowledge of CVS has increased in recent years, more and more cases are emerging.

The disorder is extraordinarily rare. While individuals of all backgrounds have been reported with the disorder, there is a higher inclination towards males (75% or more). The average age of those with Ganser syndrome is 32 and it stretches from ages 15 to 62 years old. It has been reported in children.

The disorder is apparently most common in men and prisoners, although prevalence data and familial patterns are not established.

A functional disorder is a medical condition that impairs the normal function of a bodily process, but where every part of the body looks completely normal under examination, dissection or even under a microscope. This stands in contrast to a structural disorder (in which some part of the body can be seen to be abnormal) or a psychosomatic disorder (in which symptoms are caused by psychological or psychiatric illness). Definitions vary somewhat between fields of medicine.

Generally, the mechanism that causes a functional disorder is unknown, poorly understood, or occasionally unimportant for treatment purposes. The brain or nerves are often believed to be involved. It is common that a person with one functional disorder will have others.

BFS has been reported in other African cultures, and also in Brazil, Argentina, and Ethiopian Jews. Historic higher reported prevalence among males may be due to more males being present in higher education in African countries. Studies since the 1990s have not verified gender differences. Other studies found a possible association with low socioeconomic status, an association with average or higher intelligence, and a high association with neuroticism. Individuals with BFS have been found to have problems with isolation, poor study habits, and the use of psychostimulants as well as physical changes including in muscle tension and heart rate.

Somatization disorder (also Briquet's syndrome) is a mental disorder characterized by recurring, multiple, and current, clinically significant complaints about somatic symptoms, although it is no longer considered a clinical diagnosis. It was recognized in the DSM-IV-TR classification system, but in the latest version DSM-5, it was combined with undifferentiated somatoform disorder to become "somatic symptom disorder", a diagnosis which no longer requires a specific number of somatic symptoms. ICD-10, the latest version of the International Statistical Classification of Diseases and Related Health Problems, still includes somatization syndrome.

Anumonye reported treatment success with lorazepam; others found benefit with antidepressants and relaxation exercises.

In the DSM-5 the disorder has been renamed somatic symptom disorder (SSD), and includes SSD with predominantly somatic complaints (previously referred to as somatization disorder), and SSD with pain features (previously known as pain disorder).

When untreated, the prognosis for ORS is generally poor. It is chronic, lasting many years or even decades with worsening of symptoms rather than spontaneous remission. Transformation to another psychiatric condition is unlikely, although very rarely what appears to be ORS may later manifest into schizophrenia, psychosis, mania, or major depressive disorder. The most significant risk is suicide.

When treated, the prognosis is better. In one review, the proportion of treated ORS cases which reported various outcomes were assessed. On average, the patients were followed for 21 months (range: 2 weeks to 10 years). With treatment, 30% recovered (i.e. no longer experienced ORS odor beliefs and thoughts of reference), 37% improved and in 33% there was a deterioration in the condition (including suicide) or no change from the pre-treatment status.

It is proposed that ameliorating the stress response will allow neurotransmission to return to homeostasis. Anxiolytic medications that act as 5-HT receptor agonists (in particular, 5-HT1A) together with CRH and/or cortisol antagonists (which are implicated in the stress response) are hypothesized to be an appropriate method of achieving this therapeutic response. Psychological interventions can also help to raise the threshold for stress and thereby restore the stress response to normal.

Cases have been reported from many different countries around the world. It is difficult to estimate the prevalence of ORS in the general population because data are limited and unreliable, and due to the delusional nature of the condition and the characteristic secrecy and shame.

For unknown reasons, males appear to be affected twice as commonly as females. High proportions of ORS patients are unemployed, single, and not socially active. The average age reported is around 20–21 years, with almost 60% of cases occurring in subjects under 20 in one report, although another review reported an older average age for both males (29) and females (40).

Hospitalization may be necessary during the acute phase of symptoms, and psychiatric care if the patient is a danger to self or others. A neurological consult is advised to rule out any organic cause.

This proposed subtype is characterized by depression occurring as a consequence of psychic, somatic or external stressors. Praag theorizes that if the individual's stress threshold is insufficient or overly sensitive, a prolonged stress response via the HPA axis can trigger anxiety followed by significant lowering of mood, the mechanisms of which act to reenforce each other, keeping the sufferer in a perpetual state of stress arousal, coupled with dysphoria; and that this occurs as a result of the stress response inhibiting the normal expression of neurotransmitters associated with wellbeing and pleasure. He proposes that sufferers may experience panic attacks, depersonalization and other psychic and somatic symptoms common to both anxiety and depression disorders.

According to van Praag, in Anxiety/Aggression-Driven Depression "...dysregulation of anxiety and/or aggression are primordial and mood lowering is a derivative phenomenon."

Masked depression (MD) was a proposed form of atypical depression in which somatic symptoms or behavioural disturbances dominate the clinical picture and disguise the underlying affective disorder. The concept is not currently supported by the mental health profession.

In the opinion of Allen Frances, chair of the DSM-IV task force, the DSM-5's somatic symptom disorder brings with it a risk of mislabeling a sizable proportion of the population as mentally ill. “Millions of people could be mislabeled, with the burden falling disproportionately on women, because they are more likely to be casually dismissed as ‘catastrophizers’ when presenting with physical symptoms.”

In medicine and medical anthropology, a culture-bound syndrome, culture-specific syndrome, or folk illness is a combination of psychiatric and somatic symptoms that are considered to be a recognizable disease only within a specific society or culture. There are no objective biochemical or structural alterations of body organs or functions, and the disease is not recognized in other cultures. The term "culture-bound syndrome" was included in the fourth version of the "Diagnostic and Statistical Manual of Mental Disorders" (American Psychiatric Association, 1994) which also includes a list of the most common culture-bound conditions (DSM-IV: Appendix I). Counterpart within the framework of ICD-10 () are the "culture-specific disorders" defined in Annex 2 of the "Diagnostic criteria for research".

More broadly, an epidemic that can be attributed to cultural behavior patterns or suggestion is sometimes referred to as a behavioral epidemic. As in the cases of drug or alcohol abuse or smoking, transmission can be determined by communal reinforcement as well as by person-to-person interactions. On etiological grounds, it can be difficult to distinguish the causal contribution of culture in disease from other environmental factors such as toxicity.

Psychotherapy, more specifically, cognitive behavioral therapy (CBT), is the most widely used form of treatment for Somatic symptom disorder. In 2016, a randomized 12-week study suggested steady and significant improvement in health anxiety measures with cognitive behavioral therapy compared to the control group.

CBT can help in some of the following ways:

- Learn to reduce stress

- Learn to cope with physical symptoms

- Learn to deal with depression and other psychological issues

- Improve quality of life

- Reduce preoccupation with symptom

Moreover, brief psychodynamic interpersonal psychotherapy (PIT) for patients with multisomatoform disorder has shown its long-term efficacy for improving the physical quality of life in patients with multiple, difficult-to-treat, medically unexplained symptoms.

Antidepressant medication has also been used to treat some of the symptoms of depression and anxiety that are common among people who have somatic symptom disorder. Medications will not cure somatic symptom disorder, but can help the treatment process when combined with CBT.

A 2014 meta-analysis of three small trials evaluating probiotics showed a slight improvement in management of chronic idiopathic constipation, but well-designed studies are necessary to know the true efficacy of probiotics in treating this condition.

Children with functional constipation often claim to lack the sensation of the urge to defecate, and may be conditioned to avoid doing so due to a previous painful experience. One retrospective study showed that these children did indeed have the urge to defecate using colonic manometry, and suggested behavioral modification as a treatment for functional constipation.