Scientists Studied the Genes of Pot Smokers, and the Results Are Troubling

The news: The same genes that increase your chances of developing schizophrenia could also make you more likely to use marijuana.

New research published Tuesday in the Journal of Molecular Psychiatry finds that people with genetic markers that have been associated with schizophrenia, a severe brain disorder that affects about 1% of Americans, are also more likely to smoke (or eat or vape) weed.

This is a big deal. Scientists have spent years documenting the link between schizophrenia and marijuana, but they haven't been able to nail down the nature of the relationship. While some physicians have said smoking pot can cause the disorder, whose sufferers experience paranoid thoughts and often describe hearing voices, others have said having the illness simply makes people more prone to use the drug, says King's College London genetic researcher Robert Power, who led the most recent study.

"It's been fairly well-established that there's an association," Power told Mic. "But amongst researchers there's less certainty as to which was there first."

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The new research supports the second idea. People with a variety of genetic markers that increase their chances of developing schizophrenia (so-called schizophrenia risk genes), are also more likely to use marijuana, the researchers found. The link appears to be linear: The more risk genes someone has, the more often they use and the more of the drug they use.

Basically, if your genes make you predisposed to be more at risk for schizophrenia, you could also be more likely to use marijuana.

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The details: Power's team looked at the genes and marijuana use of 2,000 healthy people, among them nearly 1,000 twins. Those with more of the schizophrenia "risk genes" were not only more likely to use marijuana, but were also more likely to use more of it than those who lacked the the risky genes or had fewer of them.

Among the twins, the relationship was even more pronounced. If both used marijuana, for example, they also had the most genetic schizophrenia risk markers. In pairs where neither used the drug, both twins had the fewest genetic markers. And in the pairs where one twin used and the other didn't, the genetic profile was mixed.

What the study does not say: The researchers did not conclude that smoking weed causes schizophrenia, or that having schizophrenia makes you smoke weed, although scientists haven't completely ruled that out.

Years of previous research show a link between marijuana and schizophrenia, but none confirm that one causes the other. A 2012 paper published in Psychological Medicine, for example, followed up on a 1987 study of 45,000 Swedish military members who had been asked if they used marijuana. Those who said they had smoked weed more than 50 times were six times more likely to have been diagnosed with schizophrenia 15 years later than those who said they had not. A 2013 study in Schizophrenia Bulletin found that teens who smoked weed daily for about three years had abnormal brain structure changes in the area related to working memory, changes that the researchers noted resembled schizophrenia-related brain abnormalities.