People at high risk of schizophrenia are more likely to smoke cannabis, a study has found.
Scientists studied more than 180,000 people worldwide using data from DNA testing website 23andMe as well as UK health records and past studies.
They found 35 genes which influence whether people are ever likely to smoke marijuana.
And the same genes also affect how likely someone is to develop schizophrenia, showing that as the likelihood of one increases, the other does, too.
The illness could actually be driving people to use cannabis in order to cope, the researchers suggest, instead of the previously-held belief that the drug can directly cause the condition.
And similar genetic links mean people are also more likely to have extroverted personalities or indulge in risky behaviour – which could drive them to smoke pot.
But the study does not give any evidence to prove that smoking marijuana does not contribute to mental health problems.
People who are genetically more likely to get schizophrenia are also more likely to use cannabis, but the drug may not directly cause the condition, according to international research
The study was carried out by researchers at the QIMR Berghofer Institute in Brisbane, Radboud University in the Netherlands and Virginia Commonwealth University in the US.
Researchers used data from past scientific studies and medical records, as well as from the website 23andMe, which offers private DNA and ancestry testing by post.
They found what they called a ‘genetic overlap’ in people’s DNA.
The overlap revealed there are some genes which increase both someone’s chance of developing schizophrenia and their tendency to smoke cannabis.
In the study the authors say the genes make people more likely to smoke cannabis by making them more likely to take risks or behave impulsively.
‘That is not a big surprise because previous studies have often shown that cannabis use and schizophrenia are associated with each other,’ said Jacqueline Vink of Radboud University.
People vulnerable to schizophrenia are more likely to use cannabis
‘Our study showed that people with a vulnerability to develop schizophrenia are at increased risk of using cannabis.’
Genetics also showed that people who are more likely to smoke cannabis have a higher chance of having a risk-taking or extroverted personality.
And many of the 35 overlapping genes also appear to make it more likely that someone will smoke cigarettes or drink alcohol.
Past research suggested drug triggered the illness
Past research has suggested that smoking cannabis can trigger schizophrenia or other mental illnesses.
But this study shows marijuana use may not be a direct cause but people may instead be using cannabis to cope with the illness, or the drug could contribute to an already-existing condition.
Professor Eske Derks from Queensland’s QIMR Berghofer Institute said: ‘This may suggest that people with schizophrenia use cannabis to cope with the symptoms.
‘These findings don’t rule out the possibility that cannabis use could also contribute to the onset of schizophrenia; however, we haven’t found any evidence to support that in this study.’
The study was published in the journal Nature Neuroscience.