The world's largest genetic study of its kind has identified 35 genes that influence whether people are likely to use cannabis.
The scientists looked at DNA samples from more than 180,000 people at QIMR Berghofer Medical Research Institute in Australia.
The institute's head of the Translational Neurogenomics Laboratory, Eske Derks who co-led the study, said the same genes which increased the use of cannabis, also influenced other personality traits and psychological conditions.
"What we found is there are shared genes that will make you more likely to start using cannabis, which will also predispose you to schizophrenia, bipolar disorder and other substances as well, such as tobacco smoking and alcohol dependence," Prof Derks said.
Previous studies showed there was an association between cannabis use and schizophrenia, she said.
"It wasn't known whether using cannabis caused the onset of schizophrenia, or whether schizophrenia caused people to use cannabis, but it was generally thought to be the former," she said.
However, this study found the genes that contributed to developing schizophrenia also made people more likely to use cannabis, Prof Derks said.
"I think it's very important we now know there's a genetic link between all these disorders, so if you were a doctor and treat a patient who has become addicted to cannabis, it's very also important to screen for other mental health disorders," she said.
The next step was to study genes which influenced the frequency and amount of cannabis people used, she said.