People who develop a cannabis use disorder share certain genetic markers, and that pattern holds across racial groups, according to the largest study of its kind.Around one-third of people who self-identify as regular cannabis users will go on to develop cannabis use disorder – the continued, regular use of the drug despite a negative impact on one’s life. People with cannabis use disorder often find it difficult to quit the drug and need higher and higher doses to feel an effect.“It’s possible that you could be only a weekend user and still meet the criteria for cannabis use disorder, but it’s pretty unlikely,” says Joel Gelernter at the Yale School of Medicine. “These are mostly much more frequent users.”The genetic link to problematic cannabis use has been explored before, but this latest research is the first to look at a large sample across different racial backgrounds.

Researchers combed the genetic information of more than 1 million individuals registered in the Million Veteran Program, which collects data from military members in the US. Their sample included a range of ancestry groups, such as European, African, East Asian and mixed race. Then, using a technique called genetic correlation, they compared variations in each person’s DNA to see if these were associated with a certain trait: in this case, cannabis use disorder.“We found that the pattern was very close to identical across the different ancestries,” says Dan Levey, also at the Yale School of Medicine. They compared variations in each person’s DNA and found that some were associated with a certain trait. For example, in people with European ancestry, strong expression of a neuronal receptor called CHRNA2 was associated with a higher risk of developing cannabis use disorder.