People with schizophrenia and some who use cannabis may have certain genes in common, according to new research.
The findings may partly explain the following, long-established link between cannabis use and psychotic disorders: The rate of people who smoke pot is higher among those who are diagnosed with schizophrenia than it is in the general population. In addition, studies have shown that using cannabis can lead to psychotic episodes or worsen a person's existing symptoms, which suggests that the substance could trigger the psychotic disorder.
But the new results suggest that part of the association between the two might actually be due to both of them being linked with the same genes, said Robert Power, a researcher at the Institute of Psychiatry at King's College London.
"To some extent, those genes that we have identified as being 'schizophrenia-inducing' are also genes that can contribute to cannabis use," Power said. [5 Psychiatric Disorders Share Common Genes]
Power and his colleagues looked at the pot-smoking habits of 2,082 healthy people as well as their genetic profiles, focusing on the genes that have been identified as related to schizophrenia.
The results showed that people who had genes linked with schizophrenia were also more likely to use cannabis, and use it more frequently, compared with people who didn't have the schizophrenia-risk genes, according to the study, published today (June 24) in the journal Molecular Psychiatry.
Schizophrenia is a psychotic disorder characterized by hallucinations, delusional beliefs and jumbled thoughts. The disorder is highly heritable — although it affects about 1 percent of the population, the risk jumps to about 10 percent for children with schizophrenic parents, and to 50 percent for twins with one twin having the disorder.
In the new study, the researcher also looked at about 1,000 twins, and found a stepwise relationship between genetic risk for schizophrenia and the likelihood of using cannabis. Those twin pairs who both reported the use of cannabis had the greatest burden of schizophrenia-risk genes; twin pairs with only one user had an intermediate level of schizophrenia risk and the lowest burden was found in pairs where neither twin reported using marijuana.
The results suggest that the potential role of cannabis in causing schizophrenia, which has inﬂuenced discussion over the legislation of the drug, may be slightly overestimated, the researchers said.
However, the findings are probably only part of a more complicated picture, Power said.
"I wouldn't say that there's no risk of cannabis causing schizophrenia, but that it's probably more complex a relationship that we are currently thinking," Power told Live Science.
It is possible that in fact both scenarios are true: that cannabis may cause schizophrenia, but also that people with schizophrenia are more likely to use cannabis for various reasons. For example, it may be because they are more likely to have a personality geared toward experimenting with drugs, Power said.
It's also possible that patients with mental illnesses may try cannabis to self-medicate. Studies have found that the drug can help alleviate some symptoms of schizophrenia, although those symptoms only become worse later.
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