In a review of A New Leaf: The End of Cannabis Prohibition, Jerome Groopman questions the alleged link:
Perhaps the most controversial and important concern around cannabinoids is whether they increase the risk of psychoses like schizophrenia. This question is most germane for adolescents and young adults. A number of studies reviewed the health records of young people in Sweden, New Zealand, and Holland who reported cannabis use, as compared to the records of those who did not. A combined or metaanalysis of results from nearly three dozen such studies linked cannabis use to later development of schizophrenia and other psychosis.
The limitation of such observational studies is that they may suggest an association but in no way prove a causal link. Indeed, the medical literature is littered with observational studies that were taken as meaningful but later overturned when randomized placebo-controlled trials were conducted. Here the Women’s Health Initiative comes to mind. This was a randomized study, using placebos as controls, that reversed some four decades of thinking about the alleged benefits of hormonal replacement therapy among postmenopausal women in preventing dementia and heart disease. No one is likely to conduct a randomized controlled trial of thousands of teenagers, assigning one group to smoke or ingest cannabis and the other group to receive placebos. The issue of marijuana as a cofactor in the development of schizophrenia and other psychosis will therefore remain unresolved.
My take on the subject is here.
The Dish 