"Despite claims that marijuana use may play a causal role in diagnosed incidences of schizophrenia, there exists no empirical evidence anywhere on Earth indicating that populations which have experienced rising rates of cannabis use have also experienced a parallel increase in rates of mental illness," Armentano said.

Most recently, a 2009 systematic review published in the scientific journal Schizophrenia Research compared trends in marijuana use and incidences of schizophrenia in the United Kingdom from 1996 to 2005. Researchers reported that the "incidence and prevalence of schizophrenia and psychoses were either stable or declining" during this period, even the use of cannabis among the general population was rising.

Armentano said that a non-causal association likely exists between marijuana use and psychosis because the symptoms of mental illness often strike early in life — at a time when young people are likely to be already experimenting with cannabis. He also speculated that some people diagnosed with psychotic disorders might be turning to cannabis after the onset of symptoms as a form of self-medication.

Ultimately, however, Armentano said that even if the latest concerns about the potential adverse effect of marijuana use are to be taken at face value, then such findings support a policy of cannabis legalization and regulation – not criminal prohibition.

"Health risks connected with pot use — when scientifically documented — should not be seen as legitimate reasons for criminal prohibition, but instead, as reasons for the plant's legal regulation." he said. "For example, we as a society don't regulate the production, use, and sale of alcohol because it is innocuous, but rather because we acknowledge that its consumption, in some situations, may pose a risk of harm. Placed in this context, today's latest warnings do little to advance the government's position in favor of tightening prohibition, and provide ample ammunition to wage for its repeal."

For more information, please contact Paul Armentano, NORML Deputy Director, at: paul@norml.org. NORML's commentary on the subject of marijuana use and mental illness is available on the NORML blog at: http://blog.norml.org.