Is the Link Between Cannabis and Schizophrenia Genetic?
We frequently hear that smoking cannabis, and particularly frequent use, can lead to developing serious psychological problems like schizophrenia. However, new research says that the link might actually be much more fascinating and complex.
Schizophrenia is a mental health condition which embodies a variety of symptoms including changes in behavior, auditory and visual hallucinations, delusions and an inability to think coherently, often due to those hallucinations. Previous research has shown that people who smoke cannabis tend to have almost double the chance of developing schizophrenia but, as yet, it isn’t clear why that might be.
This latest study, which is published this month in Molecular Psychiatry, suggests that the problem may not solely be down to cannabis use causing schizophrenia as such, but rather that people who are more likely to want to use cannabis are also more likely to have genetic markers that could lead them to develop schizophrenia. It’s a subtle but important difference and it’s worth looking at.
The Schizophrenia and Cannabis Genetic Link Study’s Methodology
Carried out by researchers at the Institute of Psychiatry, Kings College London, in partnership with several other research institutes including the Washington University School of Medicine, the research involved 2,082 unrelated healthy adults who were recruited from what’s known as Australian Twin Registry studies. This new research is in fact a cross-sectional analysis of that larger cohort study and so is digging down into the data to look at specific issues.
In the cohort study, participants were asked questions about cannabis use, including whether they used the drug or not, how old they were when they first used the drug (discounting medical use), and how many times they used the drug (again, discounting medical use).
That study also involved getting each participants’ genotype or, to put it another way, a look at their genetic make-up. For the purpose of the new analysis, the researchers then compared all that data to Swedish research samples which have identified subtle differences in our DNA (called single nucleotide polymorphisms or SNPs) that researchers believe make us more likely to develop schizophrenia. Obviously, the more of those differences are present the greater our risk of developing schizophrenia. This gives a kind of “risk score” that the researchers then used when they looked back at the answers people gave about their cannabis use.
The researchers also embarked on a second analysis to look at the risk scores among 990 twins, where about a third were identical twins. They did this to see if they could use those genetic risk scores to predict whether one or both of the twins used cannabis.
Do Cannabis Use and Schizophrenia Share a Genetic Link?
Out of the 2,082 adults included in the Australian Twin Registry study, just under half had used cannabis (48.6%). On average, people started using around age 20 and they reported using cannabis, on average, around 63 times in their lives.
What the researchers found was a significant link between how many schizophrenia-associated genetic markers someone had and whether or not they used cannabis, with the highest use among those with the highest genetic risk scores.
That said, the genetic risk factor score only predicted using cannabis a relatively small number of times, so while the risk score could predict cannabis use, it wasn’t the only or even major factor. That means that people aren’t using cannabis solely on the basis of it being an expression of their genetic risk factor for schizophrenia and could be doing so for a variety of other reasons.
You’ll remember the twin study analysis we mentioned above. Well, again, the secondary analysis showed that where the twins both reported using cannabis, they both tended to share the genetic risk factors for schizophrenia. If only one of them used cannabis, they tended to only have some of the genetic markers for schizophrenia risk, and if neither of them used cannabis, they tended to have the lowest genetic risk scores.
What Does this Research Tell Us About Cannabis and Schizophrenia?
You’ll notice one important thing here: none of the people involved in this study had actually been diagnosed with having schizophrenia. We’re talking about genetic markers and not the actual condition itself, so the researchers can’t give us a figure on how many people who tested as having used cannabis and having schizophrenia risk factors later went on to develop the condition. That’s a significant limitation. So, what we are actually looking at here is saying can we safely say that there is a genetic predisposition that leads to both cannabis use and possibly schizophrenia?
Put simply, no. This research isn’t anywhere near comprehensive enough. It does, however, suggest avenues for future research and serves to show how subtle yet significant genetic traits can be for influencing our behavior.
“Studies have consistently shown a link between cannabis use and schizophrenia. We wanted to explore whether this is because of a direct cause and effect or whether there may be shared genes which predispose individuals to both cannabis use and schizophrenia,” says Robert Power, lead author from the MRC Social, Genetic and Developmental Psychiatry (SGDP) Centre at the Institute of Psychiatry at King’s College. “Our study highlights the complex interactions between genes and environments when we talk about cannabis as a risk factor for schizophrenia. Certain environmental risks, such as cannabis use, may be more likely given an individual’s innate behaviour and personality, itself influenced by their genetic make-up. This is an important finding to consider when calculating the economic and health impact of cannabis.”
As such, and even though this research doesn’t give us any concrete conclusions yet, it is an important step to understanding the causes of schizophrenia and, hopefully, how we might prevent it from developing.
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