What's the connection, if any, between marijuana and schizophrenia? Scientific studies have, in some cases, found a tenuous link, but common sense also says that if marijuana caused schizophrenia, incidents of the chronic, severe, disabling brain disorder would have skyrocketed starting in the 1960's, alongside the rapid rise of marijuana smoking in the United States. Instead, schizophrenia rates held steady, even as millions of Americans turned on to reefer for the first time.
Now a new study, led by King's College London, and published in the scholarly journal Molecular Psychiatry, suggests that previous researchers may have misunderstood the dynamic linking marijuana and schizophrenia entirely. 

“Studies have consistently shown a link between cannabis use and schizophrenia,” according to lead author Robert Power, from the MRC Social, Genetic and Developmental Psychiatry (SGDP) Centre at the Institute of Psychiatry at King's College, "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."

The study examined 2,082 healthy individuals, including 1,011 who'd previously used cannabis. For each participant, a 'genetic risk profile' was measured, based on the number of genes related to schizophrenia they carried. The findings showed that those genetically pre-disposed to schizophrenia used cannabis more often, and in greater quantities than those without schizophrenia risk genes.

“Our study certainly does not rule out that cannabis increases the risk of schizophrenia,” according to Powers. “But it suggests that 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. Our study also 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 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."