A new study published on Monday which uses data from 23andMe found that people who are genetically at risk for developing schizophrenia are also more likely to use marijuana, which suggests the disorder itself could cause cannabis use in certain people. So it's unclear whether the drug leads to the disorder or vice versa.
The study, which was published in Nature Neuroscience, is a continuation of efforts to determine what genetic variations make people more likely to begin using cannabis. The project is called the International Cannabis Consortium. The researchers analyzed data from the DNA testing company 23andMe and also studied genetic data from other previous or ongoing studies. They examined over 180,000 individuals, which makes this the largest study of its kind to date.
According to Gizmodo, an individual's genetic code can be slightly different from someone else's in many ways, but the most common variation is a single-nucleotide polymorphism, or SNP. This is a minute change in the building blocks that make up DNA. So in one section of DNA, most people might have adenine (A), which is one of the four nucleobases that make up a nucleotide, but others might have a cytosine (C) instead.
Researchers found eight of these SNPs that were associated with life-long cannabis use. They calculated that as a whole, these variations accounted for 11 percent of the difference in whether or not an individual reported using marijuana.
They also discovered 35 genes in 16 different places across the genome that were associated with marijuana use. Several of these genes also seem to be associated with other mental health problems and personality traits. Certain variations in one gene, in particular, CADM2, seem to be associated with greater alcohol use and smoking cigarettes.
The study's lead author, Jacqueline Vink, stated, "This is not a big surprise, because previous studies have often shown that cannabis use and schizophrenia are associated with each other. However, we also studied whether this association is casual."
They used a research method called Mendelian randomization to find a possible cause-and-effect relationship between cannabis use and schizophrenia. This allows geneticists to ask whether having the genes for something directly predisposes you to something else. In this case, they found evidence that being genetically susceptible to schizophrenia made people more likely to use marijuana, possibly as a way to deal with their condition.
However, it still isn't understood exactly how schizophrenia and marijuana are connected. Other studies have found that marijuana use itself increases the risk of developing schizophrenia, especially if begun at an early age by those already at risk of mental illness.
More research is needed. For now, the researchers plan on studying whether or not there are certain genes that can predict more frequent or heavier cannabis use.