Previous studies have pointed to a link between cannabis use and schizophrenia, but it has remained unclear if this link is due to cannabis directly increasing the risk of the disorder or not.
The new results suggest that part of this association is becuase of common genes, but on the other hand, they do not rule out a causal relationship between cannabis use and schizophrenia risk.
Lead author Robert Power, of the MRC Social, Genetic and Developmental Psychiatry (SGDP) Centre at the Institute of Psychiatry at King’s, says:
“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.“
Cannabis, the most widely used illicit drug in the world, has a higher rate of use among 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 most common symptoms of schizophrenia are delusions and auditory hallucinations. Whilst the exact cause is unknown, a combination of physical, genetic, psychological and environmental factors can make people more likely to develop the disorder.
Previous studies have identified a number of genetic risk variants associated with schizophrenia, each of these slightly increasing an individual’s risk of developing the disorder.
“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.“
The King’s College study involved 2,082 healthy individuals of whom 1,011 had used cannabis. Each individual’s ‘genetic risk profile’ (the number of genes related to schizophrenia each individual carried), was measured.
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.
“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.“
R A Power, K J H Verweij, M Zuhair, G W Montgomery, A K Henders, A C Heath, P A F Madden, S E Medland, N R Wray, N G Martin. Genetic predisposition to schizophrenia associated with increased use of cannabis. Molecular Psychiatry, 2014; DOI: 10.1038/mp.2014.51
“Cannabis is the most commonly used illicit drug worldwide. With debate surrounding the legalization and control of use, investigating its health risks has become a pressing area of research. One established association is that between cannabis use and schizophrenia, a debilitating psychiatric disorder affecting ~1% of the population over their lifetime. Although considerable evidence implicates cannabis use as a component cause of schizophrenia, it remains unclear whether this is entirely due to cannabis directly raising risk of psychosis, or whether the same genes that increases psychosis risk may also increase risk of cannabis use.
In a sample of 2082 healthy individuals, we show an association between an individual’s burden of schizophrenia risk alleles and use of cannabis. This was significant both for comparing those who have ever versus never used cannabis (P=2.6 × 10−4), and for quantity of use within users (P=3.0 × 10−3). Although directly predicting only a small amount of the variance in cannabis use, these findings suggest that part of the association between schizophrenia and cannabis is due to a shared genetic aetiology. This form of gene–environment correlation is an important consideration when calculating the impact of environmental risk factors, including cannabis use.”
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