Schizophrenia And Marijuana Use May Be Linked By The Same Set Of Genes. By Shweta Iyer. medicaldaily.com.  June 24, 2014. A new study published in Molecular Psychiatry suggests that people who are genetically predisposed to developing schizophrenia may also have the propensity for cannabis use, influenced by the same set of genes. The study is a collaboration between King’s College London and the Queensland Institute of Medical Research in Australia, partly funded by the UK Medical Research Council (MRC).

Researchers believe that developing schizophrenia and using cannabis may be due to similar genes.Photo courtesy of ShutterstockFor years researchers have linked the use of cannabis as a risk factor for developing schizophrenia. Experiments showed that people who used cannabis more frequently, had greater chances of experiencing schizophrenic episodes than those who did not. But the role of genes related to schizophrenia and cannabis use was not well-established until now. This new study suggests that there may be some genetic correlation between the two but it does not rule out the use of cannabis as a factor in developing schizophrenia.
"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, according to a press report, 
For the study, researchers assessed 2,082 healthy individuals of whom 1,011 had used cannabis. Each individual’s ‘genetic risk profile’ was measured. This pointed out the number of schizophrenia related genes that each person carried. It was found that people who carried the schizophrenia gene were more likely to use cannabis, and use it in greater quantities than those who were not genetically predisposed to developing schizophrenia. 
Cannabis, more commonly known as marijuana, is a depressant drug that lowers neurotransmission levels, which means it slows down the messages travelling between the brain and the body thus depressing arousal or stimulation. Long term use of cannabis is known to cause memory loss, learning difficulties, mood swings, and lowered sex drive, among others. It is the most widely used illicit drug in the world and the WHO estimates that about 147 million people, 2.5% of the world population, consume cannabis. People suffering from schizophrenia use it more than the general population.
Schizophrenia is known to effect 1 in 100 people and people who use cannabis are about twice as likely to develop the disorder.  Although genetic mutations along with environmental factors such as exposure to certain viruses during early life, or low oxygen supply during birth are thought to cause schizophrenia, use of cannabis makes schizophrenia symptoms like delusions (false beliefs), auditory hallucinations (hearing voices), disorganized speech and behavior worse and there is a higher chance of hospitalization. Treatment is considered to be less effective and recovery rates are slow.
"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. "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."
Source- Power, R. et al. Genetic predisposition to schizophrenia associated with increased use of cannabis, Molecular Psychiatry, 2014.
Journal reference: RA Power, KJH Verweij, M Zuhair, GW Montgomery, AK Henders, AC Heath, PAF Madden, SE Medland, NR Wray y NG Martin. “Genetic predisposition to schizophrenia associated with increased use of cannabis”. Molecular Psychiatry (2014), 1–4. http://dx.doi.org/10.1038/MP.2014.51