People who regularly use marijuana are more likely to develop schizophrenia, especially those who start in their teenage years. That in itself is hardly news. In fact, it's been an established fact for a while. Only now, we might be starting to understand why exactly the two are linked.

It appears that it is (at last partly) thanks to your genes. According to a study recently published in the journal Nature Neuroscience, those who are genetically predisposed to schizophrenia are more likely to take up the habit in the first place. The study authors suggest that pot smoking may even be used by those in the early stages of the illness as a form of self-medication.

"[P]revious studies have often shown that cannabis use and schizophrenia are associated with each other. However, we also studied whether this association is causal," said Jacqueline Vink, lead author and a professor in the Behavioural Science Institute at Radboud University, the Netherlands.

"Our study showed that people with a vulnerability to develop schizophrenia are at increased risk of using cannabis."

It is not the first study to reach this conclusion. In 2014, research published in Molecular Psychology suggested genes predisposing individuals to schizophrenia may increase the likelihood of marijuana use, too. This, however, is a much larger and more comprehensive examination at the genetics involved – and it appears to support the 2014 verdict.

Done in association with the International Cannabis Consortium (which is currently looking at genetics and marijuana use) it involved more than 180,000 individuals from several genetic databanks, including the DNA test company 23andMe, the UK Biobank, and 16 smaller cohort studies.

In total, the researchers found eight single nucleotide polymorphisms (SNPs) – the tiny genetic variations between individuals located in the DNA – tied to lifetime marijuana use. Each SNP is just a single alteration in the building blocks that make up the DNA (a nucleotide), so where one person has a nucleotide cytosine (C), another may have the nucleotide thymine (T). These, the authors say, account for roughly 11 percent of the differences between people that determine whether or not they'll start using marijuana.

There were also 35 genes associated with cannabis use. The most important being CADM2, a molecule that has previously been associated with risky behavior, greater alcohol use, and certain personality traits including agreeableness and extraversion.

While the results add more weight to the idea that predisposition to schizophrenia increases the likelihood an individual will take up marijuana use (and not the other way around), it does not exclude the possibility of a reverse cause-and-effect relationship. That means marijuana use could still add to the risk of developing schizophrenia – as previous studies have suggested. 

As always, the association between drug use and mental illness is messy and complicated but now we are a little step closer to understanding how the two are linked.