Read what Professor David Nutt has to say about this.
Originally Posted by Gatts:
What we can say is that cannabis use is associated with an increased experience of psychotic disorders. That is quite a complicated thing to disentangle because, of course, the reason people take cannabis is that it produces a change in their mental state. These changes are a bit akin to being psychotic – they include distortions of perception, especially in visual and auditory perception, as well as in the way one thinks. So it can be quite hard to know whether, when you analyse the incidence of psychotic disorders with cannabis, you are simply looking at the acute effects of cannabis, as opposed to some consequence of cannabis use.
If we look on the generous side, there is a likelihood that taking cannabis, particularly if you use a lot of it, will make you more prone to having psychotic experiences. That includes schizophrenia. But schizophrenia is a relatively rare condition, so it's very hard to be sure about its causation. The analysis we came up with was that smokers of cannabis are about 2.6 times more likely to have a psychotic-like experience than non-smokers. To put that figure in proportion, you are 20 times more likely to get lung cancer if you smoke tobacco than if you don't.
The other paradox is that schizophrenia seems to be disappearing (from the general population), even though cannabis use has increased markedly in the last 30 years. So, even though skunk has been around now for 10 years, there has been no upswing in schizophrenia. In fact, where people have looked, they haven't found any evidence linking cannabis use in a population and schizophrenia.