Conclusive evidence that smoking cannabis is linked to an earlier age of onset for psychosis by up to 2.7 years has been delivered by a team of international researchers in the first ever meta-analysis of more than 20,000 individuals with mental illness.
“The study re-analyzed the results from 20,000 patients with schizophrenia or other psychotic illnesses from 83 previous studies. The study used meta-analysis — a modern statistical method — to show that an earlier onset of severe mental illness among substance users is a result of cannabis use, and cannot be explained by other factors such as alcohol use,” said Dr. Matthew Large from the University of New South Wales (UNSW) School of Psychiatry and Sydney’s Prince of Wales Hospital.
The results are based on several decades of research, and so the finding is a critical breakthrough toward the understanding of the relationship between cannabis use and psychosis, said Large. He and the other researchers concluded that cannabis use hastens schizophrenia and other psychotic disorders, possibly through an interaction between genetic and environmental factors or by disrupting brain development.
Several previous studies have established that there is a link between the use of cannabis, alcohol and other psychoactive substances and psychosis. This study, however, was aimed at revealing the particular extent to which this is caused by cannabis use alone, he said.
”Results of this study are conclusive and clarify previously conflicting evidence of a relationship between cannabis use and the earlier onset of a psychotic illness, with evidence supporting the theory that cannabis use plays a causal role in the development of psychosis in some patients,” said Large.
Large noted that there was a high rate of substance use among patients treated in mental health establishments, and individuals with schizophrenia were more likely to use substances than people in the wider community.
“The study raises the question of whether those substance users would still have gone on to develop psychosis a few years later,” said Large.
“However, even if the onset of psychosis were inevitable, an extra two or three years of psychosis-free functioning could allow many patients to achieve important developmental milestones of late adolescence and early adulthood that could lower long-term disability arising from psychotic disorders,” he said.
“The results of this study confirm the need for an ongoing public health warning about the potentially harmful effects of cannabis.”
The study is published in the journal Archives of General Psychiatry.