Prevalence & Comorbidity of Hallucinations

Based upon the literature review, it appears clearly that the prevalence of reported hallucinations has rarely been assessed in the general population.

Furthermore, the association between hallucinations and their causes namely organic pathologies and psychoactive substances was never explored in subjects of the general population.

Consequently, we plan a study:

  • to provide data on the prevalence of hallucinations in the general population of three European countries;
  • to describe their comorbidity with organic diseases, mental disorders and psychoactive substances use.

The purpose of this study was:

  • to provide additional data on the prevalence of hallucinations in the general population of three European countries,
  • and to study their association with organic diseases, mental disorders and psychoactive substances use.


The samples are comprised of respondents from three countries: the United Kingdom, Germany and Italy.

The participation rate was:

  • 79.6% (4,972 of 6,249 eligible individuals) in the UK;
  • 68.1% (4,115 of 6,047 eligible individuals) in Germany; and
  • 89.4% (3,970 of 4,442 eligible individuals) in Italy.

Overall, 13,057 subjects participates in the studies. This sample is representative of 158,690,882 inhabitants. The overall participation rate was 78%.


Aside from psychotic disorders, many other conditions may trigger hallucinatory phenomena, namely:

  • use of drugs such as cocaine and opiates,
  • brainstem pathology,
  • neurological diseases and
  • other mental disorders such as mood disorders.

Certain hallucinations can be considered normal phenomena. This is the case for hypnagogic and hypnopompic hallucinations: They were unrelated to any of these pathologies in about half of the cases. For individuals who are frightened by their hypnagogic hallucinations, the risk of a concomitant mental disorder is four times as high and the risk of narcolepsy is six times as high as in the remaining sample.

To limit problems related to the inquisitive nature of the questions about private feelings or perceptions, the questionnaire was quite extensive to ensure a smooth progression of the investigation in order to avoid an unexpected termination of the interview and to allow the differentiation between illusions and hallucinations; and the identification of mental organic and toxic disorders that may be responsible for the hallucinatory phenomena.


The results of this study show that:

  • 2% of the sample reported having frequent daytime hallucinations (visual, auditory, olfactory, haptic or gustatory hallucinations, or out-of-body experiences) and
  • 16.3% infrequent daytime hallucinations (less than once a week).

Proportions were comparable across the three countries. Use of drugs (opiates, cocaine, amphetamines, etc.) was significantly associated with almost all the types of hallucinations investigated, with the exception of auditory hallucinations. Haptic and gustatory hallucinations were significantly associated with the use of hypnotic medication. More specifically, gustatory hallucinations were more prevalent among respondents who took zopiclone. One of the main side effects of this hypnotic is the impression of a strange or a metallic taste in the mouth. Haptic hallucinations, instead, were associated with hypnotics as a whole.

Clearly, hallucinations affected individuals with a mental disorder more often than those without. About one third of mentally disordered respondents had experienced infrequent hallucinations and about 12% reported having them at least on a weekly basis. Similarly, respondents with an organic disease were more likely than others to report hallucinations.

The risks factors for hallucinations identified in this study are similar to those reported in clinical studies. Accordingly, use of drugs such as cocaine or opiates is a significant risk factor for virtually all type of hallucinations, especially for visual and tactile hallucinations that are frequent in the case of intoxication. Similarly, auditory hallucinations have been reported relative to alcohol withdrawal. Psychotic disorders were significantly associated with auditory, haptic and visual hallucinations, which are the three most common forms of hallucinations in psychotic patients.


Hypnagogic hallucinations were very frequent (24.8%) in this sample, but can be considered non-pathological in most cases (Ohayon et al., 1996). We frequently found frightening hypnagogic hallucinations associated with narcolepsy.

Hypnagogic and hypnopompic hallucinations need to be differentiated from other types of hallucinations in the sense that they occur solely in the transitional period from wake to sleep (hypnagogic) or from sleep to wake (hypnopompic).

Most individuals who experience these hallucinations know that the perception is not true. However, if an individual is terrified by what he perceives, he may attempt to escape the hallucination and severely injure himself or others.

Furthermore, there are case reports in the literature of narcolepsy being misdiagnosed as a psychotic disorder (Douglass et al., 1991; 1993; Shapiro & Spitz, 1976). This may occur if an individual complains only of frightening hypnagogic hallucinations. Moreover, as sudden sleep episodes occur in the daytime and are often preceded by hypnagogic hallucinations, distinguishing between the two diagnoses can be difficult.

The hallucinations of psychotic individuals occur at any moment of the day, including at sleep onset. It is important, therefore, to ensure that hallucinatory phenomena occur outside of sleep onset periods in order to rule out narcolepsy. The presence of hallucinations is not equivalent to schizophrenia. Life conditions such as sensory or sleep deprivation and even stress can trigger these phenomena. Hallucinations are a symptom of a psychotic disorder only when individuals firmly believe them to be true despite what the people around them say.

On a theoretical point of view, someone could ask whether hypnagogic and hypnopompic hallucinations are true hallucinatory perceptions. In our opinion, they are, even if their frequency relates them more to a dreamy state than to a pathological condition. The main difference between hypnagogic, hypnopompic and visual or auditory hallucinations relies on the significance the individual gives to the perceptual phenomenon: following the hallucinatory experience, a subject with hypnagogic or hypnopompic hallucinations knows, in most of cases, that the perceived object did not exist.

In psychotic individuals, the hallucinations become the reality and are integrated as such: the interpretations about their meaning reinforce the pathological character of the hallucination.


Olfactory and gustatory hallucinations also pose a problem. They are rather a strange perception that is often not associated with a pathological interpretation. As our results show, they are frequently associated with the use of psychoactive substances such as street drugs (cocaine, amphetamines, opiates, etc.), alcohol and medication.

Therefore, we can understand these types of hallucinations rather like a pathological effect on the sensory organ itself. Although we have found psychiatric disorders associated with these types of hallucinations, the first question to ask is how many of these psychiatric subjects also received a treatment that could explain the hallucinatory phenomena.

For some drug users, olfactory and gustatory hallucinations could be rather a dysosmia or a dysgeusia. Furthermore, haptic, gustatory hallucinations and out-of-body experiences are difficult to distinguish from illusions. In visual and auditory hallucinations, other individuals may witness the presence or absence of the hallucinatory phenomenon. In the other types of hallucinations, the hallucinatory character relies on the description and on the interpretation given to this experience.

In conclusion, hallucinations are symptoms involved in many fields of pathology. Where mental disorders are concerned, they are at the root of delusional themes. They can also be specific to certain types of drugs or neurological diseases. This study showed that the prevalence of hallucinations in the general population is not negligible.


Content of this page is extracted from:
Ohayon MM. Prevalence of hallucinations and their pathological associations in the general population. Psychiatry Res. 2000 Dec 27;97(2-3):153-64.