The risk of schizophrenia and other psychosis is small but real:
The overall additional risk to cannabis smokers is small, but measurable. One in 100 of the general population have a chance of developing severe schizophrenia; that rises to 1.4 in 100 for people who have smoked cannabis.
But the risk of developing other psychotic symptoms among people who smoke large quantities or are already prone to mental illness is significant, the researchers say.
People who smoke cannabis daily have a 200% increased risk of psychosis. They estimate that 14% of 15- to 34-year-olds currently suffering schizophrenia are ill because they smoked cannabis, a figure previously thought to be between 8% and 10%. According to the current diagnosis rates about 800 people would have been spared schizophrenia if they had not smoked cannabis.
The journal article is here.