Much of the purported link between marijuana use and psychosis is based on correlations between marijuana use and schizophrenia-like symptoms or traits, called “schizotypal personality.” But this study suggests that the effects of other drugs also used by some who use marijuana may be confounding such findings: “Lifetime use of psychomotor stimulant drugs plus ecstasy accounted for associations between cannabis and scores on the SPQ [Schizotypal Personality Questionaire] and its different subscales. Dividing groups by type of drug use revealed that those who used only cannabis and legal drugs (CLDs) (n=126) were no different from those who used only legal drugs (LDs) (n=74) but both groups scored significantly lower on the SPQ than polydrug users (n=247).”

Nichalas T. Van Dam, Mitch Earleywine and Greg DiGiacomo, “Polydrug Use, Cannabis, and Psychosis-Like Symptoms,” Human Psychopharmacology 2008 (published online ahead of print, Apr. 30, 2008)
Free abstract and full text for a fee available at