David M. Fergusson, PhD, Research Professor in the Department of Psychological Medicine at the Christchurch School of Medicine & Health Sciences at the University of Otago in New Zealand, et al., wrote in their article titled "Tests of Causal Linkages Between Cannabis Use and Psychotic Symptoms," published in Mar. 2005 in Addiction:

“Daily users of cannabis had rates of psychotic symptoms that were between 1.6 and 1.8 times higher than non-users of cannabis….

The results of the present study add to a growing body of evidence suggesting that regular cannabis use may increase risks of psychosis.

The present study suggests that:

  1. the association between cannabis use and psychotic symptoms is unlikely to be due to confounding factors; and
  2. the direction of causality is from cannabis use to psychotic symptoms.”
Mar. 2005