Supporting+Evidence

=**__ Supporting Evidence __**=

Cross-sectional surveys have found that rates of cannabis use are approximately twice as high among people with schizophrenia compared to people in the general population (van Os et al, 2002). Surveys of patients with psychotic illnesses from London England have found that between 20 and 40% report lifetime cannabis use (Duke et al, 2001). But, are these higher rates of cannabis use among people with schizophrenia a consequence or a cause of the coniditon? In a follow-up study to one of the first major studies on whether cannabis use predisposed one to develop schizophrenia in the Swedan conscript study, showed that heavy cannabis users by the age of 18 years old were 6.7 times more likely than non-users to be diagnosed with schizophrenia 27 years later (Zammit et al, 2002).

__**NEMESIS Study **__ A study conducted by the Netherlands Mental Health Survey and Incidence Study (NEMESIS) (van Os et al, 2002) examined the effect of cannabis use on self-reported psychotic symptoms among the general population. They used 4045 psychosis free people and 59 with psychotic disorders, over a 3 year period. Individuals using cannabis at baseline were almost 3 times more likely to develop psychotic symptoms at follow-up, even after adjusting for a range of clinically significant factors, ie. race, gender, ethnicity. Researchers also found that lifetime history of cannabis use at baseline, compared to at follow-up, was a stronger predictor of psychosis 3 years later. This means that the correlation between cannabis and psychosis is not just a result of short-term use of cannabis leasing to acute psychosis.

__**Christchurch Health and Development Study **__ The Christchurch Health and Development Study (Fergussin et al, 2003) is a study that has followed its cohort over 20 years by examining the association between cannabis and psychotic symptoms at ages 18-21 years. Researchers found that those who met the criteria for cannabis dependence disorder at 18 years old had a 3.7 fold increase risk of psychotic symptoms compared to those without. Risk of psychotic symptoms at the age of 21 years was 2.3 times higher for those who had cannabis dependency disorder. After they controlled for many confounding variables; like anxiety disorders, childhood sexual or physical abuse, and psychotic symptoms at previous assessment, association remained significant at 21 years old.

__**Schizophrenic-Spectrum Disorders **__ In a recent study examining cannabis-induced psychosis and subsequent schizophrenia disorders in 535 cases (2005). Arendt, Rosenberg, Foldager, Perto, and Munk-Jorgensen (2005) found that of the sample of individuals who were treated with cannabis-induced psychotic symptoms, 44.5% of the individuals were diagnosed with schizophrenia-spectrum disorders. The researchers compared this experimental group to a control group of individuals who only presented with schizophrenia-spectrum disorders for the first time and no history of cannabis-induced psychosis. Arendt (2005) found that the development of schizophrenia disorders was often delayed and in 47.1% of people received a diagnosis more than a year after getting treatment for cannabis-induced psychosis. Patients also developed schizophrenia at an earlier age than the control group. This study shows that cannabis-induced psychosis is of real clinical significance and is something that needs to be researched further, but for the time being if you are experiencing psychosis due to your cannabis use you are at greater risk to develop schizophrenia.

__**Genetic Evidence **__ Obviously not all patients who have schizophrenia use cannabis and not all cannabis users develop schizophrenia. Therefore we cannot say that cannabis use is a definite stand alone cause of schizophrenia, but there is a correlation. Cannabis is codependent on some other factors in order to have any causal influence on risk for developing psychosis. McGuire et al. (1995) found that the relatives of patients with acute psychosis who tested positive for cannabis use had 10 times higher morbid risk for schizophrenia than those who did not have relatives with schizophrenia. Similarly, Henquet et al. (2005) found that the 3.5 year risk of developing psychotic symptoms was 2.1% in young people who used cannabis in the absence of psychosis liability, but found that 51% of people who both used cannabis and psychosis liability. Psychosis liability was measured by a psychosis proneness scale shown to be sensitive to familial transmission of psychosis liability.

A recent study that showed a direct link to genetic risk factors influencing the development of psychosis was conducted by Caspi et al. (2005). They showed that a functional polymorphism in catechol-O-methyltransferase (COMT) gene moderates the effect of adolescent cannabis use on risk for adult psychosis. Caspi et al. (2005) found that the individual homozygous for the COMT valine allele are most likely to show psychotic symptoms and to develop schizophrenic-spectrum disorder after adolescent cannabis use.

Genetic evidence shows that if you are biologically predisposed to developing schizophrenia or psychosis symptoms and you are a cannabis user than your chances of developing schizophrenia dramatically increases.