First of all, the CPS statement is based on the scientific literature, not on belief but rather on the facts that we have at the time we're reviewing the data. As I mentioned in my brief, the relationship between psychosis and cannabis is complicated.
My understanding of the state of the literature is that we can't say with 100% certainty that it's causative, in the sense that only cannabis use in a young person regularly over years would for sure result in their developing a psychotic illness; however, based on many studies we know that there are a number of different types of psychotic outcomes.
We know from rigorous research that one of the side effects of using cannabis can be an acute psychotic episode. Some youth experience that when they're high. There is another type of acute psychotic episode that can last days to weeks, and often those youth end up coming to emergency rooms, being seen because the psychotic symptoms have lasted beyond the high or the use. We know from research that those youth, if they're followed, have an increased likelihood of about 50% of having another psychotic episode in the future that is not related to cannabis.
I'm not a schizophrenia expert, but from reviewing the literature there is definitely a link between regular, ongoing cannabis use that starts young and continues, and the development of schizophrenia. As I mentioned in my brief, however, it's not that alone. Judging from the literature, for example, the risk of having a psychotic illness in an adult is about 1%. Add regular, ongoing cannabis and that doubles the risk to 2%, which sounds quite low if we think 2% versus 1%. However, if you have a family member who has suffered from schizophrenia or a psychotic illness and your risk as an individual is much higher, then doubling the risk with cannabis becomes significant.