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Schizofrenia: L'uso di cannabis aumenta il rischio di schizofrenia?

Schizophrenia: Does cannabis use increase the risk for schizophrenia?


The use of cannabis may be a for the development of schizophrenia, a type of psychosis. It is currently assumed that cannabis doubles the risk (or increases the risk by 2) if heavily used in adolescence. There are other factors that increase schizophrenia risk. For example having grown up in a big city also increases the risk by about 2 compared to having grown up in the countryside. This small increase in risk means that 1 to 2 out of 100 heavy cannabis users and 1 to 2 out of 100 city dwellers will develop schizophrenia during their lifetime, compared to 0.5 to 1 out of 100 people without any .

What is psychosis?

Psychosis is a serious medical condition of unknown origin. It refers to an abnormal condition of the mind, and is a generic psychiatric term for a mental state often described as involving a “loss of contact with reality”. The term “psychosis” is most often used as an “umbrella term” instead as a distinct diagnosis. People suffering from psychosis are described as psychotic. Psychosis is given to the more severe forms of psychiatric disorder, during which hallucinations and delusions and impaired insight may occur. Although there are drugs available that can ameliorate some of the symptoms, the disorder can not be cured. Schizophrenia is a special form of a psychotic disorder.

What is schizophrenia?

Schizophrenia is a characterized by a breakdown of thought processes and by poor emotional responsiveness. It most commonly manifests itself as auditory hallucinations, paranoid beliefs, or disorganized speech and thinking. Hallucinations are perceptions in a conscious and awake state in the absence of external stimuli, which have qualities of real perception. For example, schizophrenics may hear voices in the absence of any voices. Beliefs, which are called delusions, are associated with strong conviction despite evidence to the contrary. For example, somebody may believe that he is an important historical personality such as Jesus or Napoleon. Often deficits of normal emotional responses are associated such as flat or blunted affect and emotion, poverty of speech, inability to experience pleasure, and lack of motivation. Depending on the clinical schizophrenia can be classified in different subtypes.

How prevalent is schizophrenia? 

About 15 to 20 new cases per 100.000 inhabitants of Western countries develop schizophrenia every year. The onset of symptoms typically occurs in young adulthood between the age of 18 and 35, and about 0.5 to 1.0 per cent of all citizens of Europe and North America develop schizophrenia during their lifetime. Delusions, thought disorders (e.g. thought broadcasting), and acoustic hallucinations are preliminary employed in diagnosis. These symptoms are often highly dramatic and dangerous, but these so-called “positive symptoms” normally improve over the years. On the other hand, the “negative symptoms” such as depression, inability to make social contacts, impoverishment of feelings often remain, resulting in psychosocial problems and unemployment.

What causes schizophrenia?

Both genes and the environment play a role in the development of schizophrenia. Certain variants of genes are associated with a higher risk of schizophrenia. This may explain, why schizophrenia is observed more often in some families compared to others. However, these genetic variants do not cause the , but play a role in the disposition to the . Somebody, who has a first-degree relative (parents, brother, sister) with schizophrenia, has a risk of 6.5 per cent to also develop the during lifetime. This means that of 100 first-degree relatives of people with schizophrenia 6 to 7 also develop the .
Environmental risk factors that have been established are pregnancy complications including stress, infections and malnutrition of the mother, birth complications, growing up in a large city, low but normal IQ (intelligence quotient), and consumption including cannabis use. Other factors, which may play an important role, are social isolation, family dysfunction and other heavily distressing factors. People with schizophrenia in the northern hemisphere are more likely to have been born in winter or spring compared to summer and autumn.

How to prevent schizophrenia?

Since risk factors are only associated with a relatively low increase of risk, they cannot be used for early detection and prevention of schizophrenia (Klosterkötter 2008). On the other hand, it is desirable to detect schizophrenia at an early stage, since early detection and early treatment is associated with a more favourable course of the disease, less depression and less suicide. Thus, prevention efforts and programs are concentrated on the detection of risk symptoms during the so-called prodromal state (early warning signs) and on making a correct diagnosis after outbreak of the disease as early as possible.
In about three quarters of all cases the outbreak of schizophrenia is preceded by a prodromal state for an average of five years. During this period the person may have thought disturbances, unusual experiences of perception, paranoid ideas, decreased ability to discriminate between ideas and perception, fantasy and true memories, and similar symptoms several times a week. There is a high risk for people experiencing prodromal symptoms to develop psychotic symptoms and psychotic episodes finally leading to schizophrenia.

What is the role of cannabis in the development of schizophrenia?

In a review of seven longitudinal studies on the association between cannabis use and schizophrenia researchers found that individuals who had ever used cannabis had an increased risk of psychosis or psychotic symptoms of 41 per cent compared to individuals who had never used cannabis. In longitudinal studies a large number of people are followed for several years, ideally from birth to adulthood, to identify for example causes of diseases or protective factors against diseases. Frequent cannabis users had twice the risk of non-users (odds ratio: 2.09) (Moore et al. 2007). Researchers noted that the uncertainty about whether cannabis causes psychosis is unlikely to be resolved by further longitudinal studies. It is most likely that cannabis use precipitates schizophrenia in individuals who are vulnerable because of a personal or family history of schizophrenia (Degenhardt and Hall 2006).
It is difficult to prove that cannabis is indeed a causal factor in the development of schizophrenia, since the association may be non-causal, at least in part. For example, some people with schizophrenia may self-medicate with cannabis to treat some of their symptoms, especially negative symptoms. However, there is increasing evidence from long-term epidemiological studies that cannabis plays a causal role.

What is the role of in the treatment of schizophrenia?

There are two published case series, which demonstrate that cannabis or THC may be of therapeutic value in some cases of schizophrenia, who do not respond to conventional medication (Schwarcz et al. 2009,  Schwarcz et al. 2010). The authors of these reports assume that with regard to brain physiology the cause of schizophrenia in these patients may differ from other patients with schizophrenia, who respond to conventional anti-psychotic medication, that these patients may suffer from low endocannabinoid brain function.
There is clinical evidence that the natural plant cannabidiol (CBD) at a daily dose of 800 mg may be as effective as conventional medication in the treatment of schizophrenia (Leweke et al. 2012). CBD is known to decrease or abolish the of THC. The treatment with CBD is associated with an increase in anandamide blood levels, and this increase is thought to be responsible for symptom improvement.

Why using a vaporizer is healthier and better than smoking cannabis

About the author
Dr Franjo Grotenhermen is chairman of the German Association for Cannabis as Medicine (ACM) and executive director of the International Association for Cannabinoid Medicines (IACM). He is working for the nova-Institut in Huerth/Rhineland, Germany.


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