The same genes that predispose people to schizophrenia also make them more likely to use cannabis, say scientists, after studying over 1000 Australian drug users.
Previous studies have associated dope use with schizophrenia. But a British study published in the journal Molecular Psychiatry, released this week, found the link may be due to common genes and might not be a causal relationship where cannabis use leads to increased schizophrenia risk.
Cannabis is the most commonly used illicit drug worldwide and use of the herb is much higher among schizophrenic patients than in the general population.
"The relationship between schizophrenia and cannabis use may be more complicated than it initially seems," said Robert Power, who led the study at King's College London.
The study of 2082 healthy Australians - 1011 of whom had used cannabis - measured each person's genetic risk profile, noting the number of genes related to schizophrenia.
The results suggested that those with an increased genetic predisposition to schizophrenia were both more likely to use cannabis and to use it in greater quantities.
"This is not to say that there is no causal relationship between use of cannabis and risk of schizophrenia, but it does establish that at least part of the association may be due to causal relationship in the opposite direction," he said.
Dr Matthew Large, from the School of Psychiatry at the University of NSW, said the study contributes to the understanding of the association between cannabis and schizophrenia.
"This study demonstrates what has long been suspected, that there are common factors that may underlie both cannabis-use and psychosis," he said.
"Historically it has been assumed that people with psychosis smoke cannabis to alleviate their symptoms - we now know this is incorrect.
Since the 1990s there has been a large body of evidence that psychosis can be precipitated by cannabis use, almost certainly to the extent of causing some cases, he said.
"However, a causal association never explained why more than half of all people with psychosis smoke cannabis.
"The presence of a shared genetic vulnerability for psychosis could, if replicated, add greatly to our understanding of both addiction and psychosis."