Study weeds out marijuana-psychosis link

Teenagers who smoke marijuana are twice as likely to develop psychosis by their 20s as their peers who avoid the drug, Australian research shows. The younger people were when they started smoking marijuana, the more likely they were to develop non-affective psychosis, hallucinations and delusions, the study of almost 4000 people found. By analysing a subgroup of 228 sibling pairs, researchers were able to reduce the impact of genetic factors, which have beleaguered other studies of psychosis in marijuana users. People who had started using cannabis when they were aged around 15 were twice as likely to develop non-affective psychosis, the study found. Overall, 65 participants received a diagnosis of non-affective psychosis, 60% of whom reported having used cannabis. Writing in the Archives of General Psychiatry, the authors said their findings bolstered the theory that early cannabis use was a risk-modifying factor for psychosis in young adults. However, the relationship between psychosis and cannabis use was "by no means simple". "Those individuals who were vulnerable to psychosis were more likely to commence cannabis use, which could then subsequently contribute to an increased risk of conversion to a non-affective psychotic disorder," the authors said. Professor George Patton, a psychiatric epidemiologist at the Murdoch Children's Research Institute in Melbourne, said the study added to mounting evidence that early adolescent and high-dose use of cannabis during the teenage years predicted a range of mental health problems. "I think we can be confident that the researchers are picking up significant psychiatric morbidity," he said, despite the study's use of questionnaires rather than clinical assessment, to determine psychosis. Dr Alex Wodak, director of the Alcohol and Drug Service at St Vincent's Hospital in Sydney, said policies regarding cannabis should be based on the assumption that it caused psychosis, even though the absolute risk was very small. However, the contribution that alcohol and amphetamines made to mental illness "dwarfs any contribution that cannabis might make". An advocate for cannabis legalisation, Dr Wodak argued that the chances of delaying initiation and reducing consumption "would be better if cannabis was taxed and regulated rather than provided by Al Capone"....