Smoking cannabis on a regular basis can more than double the chances of suffering psychotic illnesses such as schizophrenia, a new British research study has shown.
Any use of cannabis , which means even taking the drug just once, was associated with a 41 per cent greater risk.
Scientists in Britain pooled data from 35 different studies which together showed a clear dose-response effect.
Cannabis: can double risk of psychotic illnesses, according to new study
People who smoked the most cannabis were the most likely to suffer a psychotic breakdown marked by delusions, hallucinations or disordered thoughts. For frequent users, the risk rose to between 50 per cent and 200 per cent.
The research, published today in the Lancetmedical journal, implies that 14 per cent of psychotic illness affecting young adults could be prevented if they did not smoke cannabis.
About 20 per cent of young adults across Europe claim to take cannabis at least once a week, and around 40 per cent are believed to have tried it at some time in their lives.
The report's authors said there was no definite evidence cannabis triggered psychosis, only that an association existed.
Prof Glyn Lewis, from the University of Bristol, said: "It is difficult to be certain about whether cannabis use causes psychotic illnesses such as schizophrenia. . . . However, all the studies have found an association and it seems appropriate to warn members of the public about the possible risk."
Currently, there was little to indicate that cannabis use increased the risk of depression, suicidal thoughts or anxiety, said the researchers.