A recent review concluded that the use of cannabis increases the risk of psychotic outcomes. A model of the association between marijuana use and schizophrenia showed that cases of the condition would increase from 1990 onwards.

The model is based on three factors: Frequent cannabis users had increased risk of psychotic outcomes compared to those who have never used cannabis; the substantial rise in UK cannabis use from the mid-1970s; and the elevated risk of 20 years from the first use of cannabis.

The paper examines trends in the UK, for the number of cases of schizophrenia and psychoses, compared to diagnosed cases from 1996 to 2005. The study overlooked about 600,000 patients each year, representing about 2.3% of the UK population aged from 16 to 44.

The study didn't find any evidence of increasing schizophrenia or psychoses in the general population from 1996 to 2006.

[via NCBI]