In a study of 185,000 people, 35 genes were found in which variation influences cannabis use. Those genes can also be found in heavy drinkers and people with schizophrenia. People who have smoked cannabis have, more then people who don't smoke cannabis, gene variants in their body cells that can also be found in people who smoke, are addicted to alcohol or are risky.

Those genes are also found in people who are open to new experiences, who are carefree, not so meticulous, not neurotic and who have a high education. And they are gene variants that are more common in people with schizophrenia and other mental illnesses. Genes that influence cannabis use, in short, also affect personality and mental health. This is what a large group of researchers, united in the International Cannabis Consortium and led by Radboud University Nijmegen, write in an article published in Nature Neuroscience.

They base these conclusions on research into the genetic data of almost 185,000 people. Nearly 43 percent of those people had ever used cannabis in their lives. One attempt was enough to count as a user. The researchers found 35 genes in which variation affects cannabis use. The differences in these genes accounted for 11 percent of the difference between users and non-users. The influence of genes is therefore not very large. This research also leads to gene sequences that increase the risk of schizophrenia, and thus to hallucinations, for example, also increase the chance of cannabis smoking.

There has been a heated debate for years, also in government, about the cause and effect of cannabis use and schizophrenia. If cannabis promotes schizophrenia, this could be a reason to discourage cannabis use more strongly. But there are also indications that people with a predisposition to schizophrenia are blocking their first unrest with cannabis. In these cases cannabis is being used as self-medication. The latter is most likely, the researchers write.

With a new technique (mendelian randomization) they investigated whether there is cause and effect - and whether the cannabis or the schizophrenia predisposition then is the cause. The most probable outcome is that schizophrenia predisposition results in cannabis use, but this is no rock solid conclusion yet.

Carefree and impulsive

Two of the 35 genes found seem to have the greatest influence. Those two were also found in smaller genetic studies on cannabis use. The risk-increasing variants of one of the two genes (the CADM2 gene) have also been found in people who are less anxious, neurotic and conscientious. They are carefree and impulsive. The risky variant of the other gene (NCAM1) has previously been found in people with mood disorders and schizophrenia.

A set of adjacent genes that have not previously been linked to cannabis are known from research on autism, schizophrenia, overweight and asthma.

A weakness of this study, according to the researchers, is that even people who once smoked cannabis are fully counted as 'users'. A follow-up study looks at genes that play a role in the quantity and frequency of cannabis use, writes Radboud University in a press release.