In a statement submitted to Parliament this Thursday, the New Zealand Ministry of Health told a select committee inquiry into the mental health effects of marijuana that, "Overall, the current public health risks of cannabis use are small to moderate in size, and are less than the public health risk of tobacco or alcohol use." While it noted that large amounts of marijuana caused acute temporary impairment to thinking, and that some forms of mental illness, such as schizophrenia, could be exacerbated by marijuana use, the statement also said that research shows that even long-term, heavy use of marijuana does not cause great damage to cognition, but that the vast majority of marijuana users in New Zealand use the drug only occasionally.
The Ministry's statement largely squares with the conclusions of more than a dozen blue-ribbon reports commissioned by several governments over the past one hundred years, including the Commission of the Australian Government in 1977, the National Commission on Marihuana and Drug Abuse convened by U.S. President Richard Nixon in 1972, and the Report by the Dutch Government in 1995.
In an interview in the New Zealand Dominion, New Zealand Drug Policy Forum Trust director and DRCNet advisory board member Dr. David Hadorn said that the criminalization of marijuana causes more harm to the mental health of New Zealanders than the drug itself. "Creating a climate of criminality around cannabis use," he said, "insures that the relatively few people who develop problems are less likely to seek help. This sets off a spiral of alienation, marginalisation and anti-social behaviour, which too often can culminate in criminality, mental illness and violence."
(A number of the reports cited have been re-published in DRCNet's Online Drug Policy Library; the are listed at http://www.druglibrary.org/schaffer/, Major Studies link.