The study found that participants who used cannabis, but no other illicit substances did not score higher on a Schizotypal Personality Questionnaire (SPQ) compared to those respondents who used legal drugs only. However, those participants who used cannabis and other illicit drugs such as ecstasy and cocaine did score higher on the SQP tests.

The study's results contradict widely reported news reports alleging that marijuana use is associated with a much higher risk of psychosis.

"Some previous work on the link between [cannabis] use and psychotic symptoms has not controlled for other drug use at all," said NORML Advisory Board member Mitch Earleywine, who co-authored the study. "Other studies only use rough measures of lifetime use of a few drugs. We focused on nine different drugs and emphasized the stimulants, which work in a neurotransmitter system linked closely to psychosis."

A prior review of cannabis and psychosis conducted by Britain's Advisory Council on the Misuse of Drugs (ACMD) stated in a 2006 release that, "the current evidence suggests, at worst, that using cannabis increases lifetime risk of developing schizophrenia by one percent."

Earleywine concurs with that assessment. "In my opinion, if cannabis has any impact on psychosis at all it would only appear in folks who have a genetic predisposition who also use heavily early in life," he said.

NORML published a white paper in May of 2007, which called for the regulation of cannabis as a means to discourage its use by patients who may be predisposed to certain mental illnesses.

Advocates for tougher marijuana laws in Britain have been trumpeting the pot-and-psychosis link as a major impetus for Prime Minister Gordon Brown's expected move to elevate cannabis from a Class C to a Class B drug. The ACMD declined to back the move in a recent meeting on April 4, with 20 out of the panel's 23 experts deciding there was not sufficient new scientific evidence to justify a change.

Full text of the study, "Polydrug Use, Cannabis Use, and Psychosis-Like Symptoms," appeared in Human Psychopharmacology, Aug. 23, 2008 -
http://www.ncbi.nlm.nih.gov/pubmed/18449850

Cannabis, Mental Health and Context: The Case For Regulation
http://norml.org/index.cfm?Group_ID=6798