There is growing evidence that cannabis use is a cause of schizophrenia – a debilitating psychiatric illness that affects around 1% of people at some point in their lives – and this shows the risk of developing the disorder is double among cannabis users. Now a new study led by King’s College London, UK, also finds increased cannabis use and schizophrenia may have genes in common.

Cannabis or marijuana is naturally occurring – it comes from the hemp plant Cannabis sativa – and contains a mind-altering chemical called delta-9-tetrahydrocannabinol, more commonly known as THC.

Much debate surrounds the legalization and control of cannabis, the most commonly used illicit drug in the world, putting more and more pressure on researchers to investigate the health risks of its use.

Writing in Molecular Psychiatry, the researchers explain how their study helps clear up an area of confusion: whether the link between cannabis use and schizophrenia is entirely due to use of the drug, or whether the same genes that increase psychosis risk may also increase risk of cannabis use.

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Participants whose genetic risk profile predisposed them to schizophrenia were more likely to use cannabis those who did not carry schizophrenia risk genes.

Schizophrenia’s most common symptoms include delusions – or false beliefs, and auditory hallucinations – or hearing voices. It is not clear exactly what causes the disorder, but scientists believe a combination of physical, genetic, psychological and environmental factors play a strong role in its development.

Researchers have already identified a number of genes linked to schizophrenia, variants of which each slightly increase the risk of developing it.

For their study, lead author Robert Power, of the MRC Social, Genetic & Developmental Psychiatry Centre, in the Institute of Psychiatry at King’s, and colleagues investigated a sample of just over 2,000 healthy individuals, around half of whom had used cannabis.

From the number of gene variants linked to schizophrenia that they carried, the team assigned each participant a “genetic risk profile” and compared it to cannabis use.

The results showed that participants whose genetic risk profile predisposed them to schizophrenia were more likely to use cannabis – and to use it more – than those who did not carry schizophrenia risk genes.

Mr. Power points out that the findings do not rule out that cannabis use may directly increase the risk of schizophrenia, but instead suggest “there is likely to be an association in the other direction as well – that a predisposition to schizophrenia also increases your likelihood of cannabis use.”

He suggests the study highlights the complexities of gene-environment interaction in the context of cannabis use and schizophrenia:

Certain environmental risks, such as cannabis use, may be more likely given an individual’s innate behavior 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.”

In December 2013, Medical News Today learned that use of cannabis is linked to schizophrenia-related brain changes in the thalamus – an area important for learning, memory and communication. The study, published in the journal Schizophrenia Bulletin, found that the brain abnormalities persisted long after people stopped using cannabis.