Genes that increase the risk of developing schizophrenia may also increase the likelihood of using cannabis, according to a new study.

Previous studies have identified a link between cannabis use and schizophrenia, but it has remained unclear whether this association is due to cannabis directly increasing the risk of the disorder.

The new study suggests that part of this association is due to common genes, but does not rule out a causal relationship between cannabis use and schizophrenia risk.
The study is a collaboration between King's College London and the Queensland Institute of Medical Research in Australia.

"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," said Robert Power, lead author from the Institute of Psychiatry at King's.

Cannabis is the most widely used illicit drug in the world, and its use is higher amongst people with schizophrenia than in the general population.

Schizophrenia affects approximately 1 in 100 people and people who use cannabis are about twice as likely to develop the disorder.

The new study included 2,082 healthy individuals of whom 1,011 had used cannabis. Each individual's 'genetic risk profile' was measured - that is, the number of genes related to schizophrenia each individual carried.

The researchers found that people genetically pre-disposed to schizophrenia were more likely to use cannabis, and use it in greater quantities than those who did not possess schizophrenia risk genes.

"We know that cannabis increases the risk of schizophrenia. Our study certainly does not rule this out, but it suggests that there is likely to be an association in the other direction as well - that a pre-disposition to schizophrenia also increases your likelihood of cannabis use," Power said.

The study was published in the journal Molecular Psychiatry.