Joseph M. Pierre, MD, Co-Chief of the Schizophrenia Treatment Unit at the Veterans Administration (VA) West Los Angeles Healthcare Center, wrote in his Sep. 2011 article "Cannabis, Synthetic Cannabinoids, and Psychosis Risk: What the Evidence Says" in Current Psychiatry:

“There are several ways to explain the link between cannabis use and psychosis, and a causal relationship has not yet been firmly established. Current evidence supports that cannabis is a ‘component cause’ of chronic psychosis, meaning although neither necessary nor sufficient, cannabis use at a young age increases the likelihood of developing schizophrenia or other psychotic disorders. The overall magnitude of risk appears to be modest, and cannabis use is only 1 of myriad factors that increase the risk of psychosis. Furthermore, most cannabis users do not develop psychosis. However, the risk associated with cannabis occurs during a vulnerable time of development and is modifiable.”

Sep. 2011