Drug taking and addiction can be influenced both by people’s genes and by their environment, which makes them extremely difficult to study, says Levey. But the team was able to build on data from previous work² by including genetic information from additional sources, predominantly the Million Veteran Program — a US-based biobank with a large genetic database that aims to improve health care for former military service members. The analysis encompassed multiple ethnic groups, a first for a genetic study looking at cannabis misuse.

As well as identifying regions of the genome that might be involved, the researchers saw a bi-directional link between excessive cannabis use and schizophrenia, meaning that the two conditions can influence each other. This finding is intriguing, says Marta Di Forti, a psychiatrist-scientist at King’s College London. Cannabis use “is the most preventable risk factor” for schizophrenia, she says, adding that the type of genetic data examined in the study could be used in future to identify and support people at increased risk of developing psychiatric disorders through cannabis use.

More information about the biological mechanisms that connect cannabis use with health conditions will provide a better evidence base for policy and medical practice, says study co-author Joel Gelernter, a psychiatric geneticist at Yale University.

He adds that researchers need to continue to build on this knowledge and use the data to understand the health risks of both medical and recreational cannabis use. Di Forti agrees, adding that it will also be important to investigate the specifics of how the drug is administered, and how much of the psychoactive component tetrahydrocannabinol (THC) users consume.