Since the early 1990s, researchers have been telling us that smoking marijuana can lead to schizophrenia. But a new study published in the Journal of Molecular Psychiatry has found that the association between cannabis and psychosis may be a result of overlapping genes, rather than cannabis being a direct cause of psychosis.
The study of over 1000 users found the same genes that increase the chance of developing schizophrenia also make the carrier more likely to smoke pot — and more of it — than users who don’t carry the genes.
Robert Power from the Institute of Psychiatry at King's College in London, who led the study, said the findings don’t completely rule out the link between cannabis use and developing schizophrenia, but suggest the relationship is more complex than simply cause-and-effect.
Whether or not we choose to smoke weed is not entirely random, or just down to individual choice.
''Individuals do not seek out experiences randomly, and part of their decisions for which environments to experience is determined by their genetics,'' he told VICE News.
''We see an association between an environment and a disease, like cannabis and schizophrenia. Other studies have shown that cannabis use is to some extent a heritable trait. This is generally undisputed, though it is worth noting that almost all behavioural traits are to some extent determined by your genes — it’s just a matter of how much.”
So, just like you can inherit the genes that cause cancer, you can inherit a genetic liking for weed, and a predisposition to developing schizophrenia.
Scientists have known for years that schizophrenia has an inheritable component by studying sets of non-identical and identical twins. If one identical twin has schizophrenia, there is a 50 percent chance the other twin will, as well.
Dr. Matthew Large from the School of Psychiatry at the University of New South Wales told VICE News that other mental disorders, like anxiety and bipolar disorder, have varying degrees of heritability.
The evolution of DNA technology and the ability to map the human genome have given scientists even more insight into the genetic factors of mental disease.
''We can now look at the entire genome of whole groups of people. What this has found is that there are hundreds of genes, each of which makes schizophrenia and other disorders a little more likely,” he said.
The type of weed you smoke — for instance, strong hydroponically grown strains, the age you start, and the amount you smoke, can also help determine your risk of psychosis.
Professor John McGrath from the Queensland Brain Institute at University of Queensland in Australia said if you started using pot in your teens you still have a small but increased risk of developing schizophrenia, as opposed to a non-smoker.
Dr. Large says the risk pot poses to young teenagers actually bolsters the cause to legalize cannabis.
"We generally allow people to do things that are silly if they know what they are doing,” he said. “At present, young people who often start smoking before they are 16 have no way of accessing accurate information about cannabis because it comes with no health warnings.”
''One beneficial effect of legalizing cannabis would be that it could be marked with appropriate health warnings,” he said.
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