The relationship between marijuana use and schizophrenia has been in the news lately after Malcolm Gladwell wrote in The New Yorker that cannabis can lead to the mental illness. But that's not really what science is saying about the issue.
The New York Times published a lengthy article examining the idea of marijuana use and schizophrenia. The big conclusion is that while there is a correlation between schizophrenia and marijuana use, there's virtually no evidence showing that consuming cannabis will lead to schizophrenia, speed up the process, or anything of the kind.
There are numerous studies showing that people with schizophrenia are regular marijuana users more than the general public. But the same is also true of people with schizophrenia and alcohol or cigarettes. In fact, there are studies showing that smoking cigarettes at a young age is actually a much better predictor of whether or not someone will develop schizophrenia. But you don't hear people talking about that. They only focus on marijuana.
It's also not entirely clear how marijuana use would lead to schizophrenia. The biggest theory is that when someone is a late teen or early adult, their brains go through a pruning phase where they basically get rid of "unneeded or redundant connections between brain cells." This process largely takes places in the prefrontal cortex, where there are many CB1 receptors that become activated by marijuana use. Some scientists say that any alterations or disturbances to the pruning process could increase the risk of schizophrenia, and that using marijuana and activating those CB1 receptors could do that. However, this is really more of a hypothesis, and there are no studies showing that using marijuana does in fact alter the pruning process.
So there is a correlation between marijuana use and schizophrenia, but there's no evidence that cannabis can cause the mental illness. This isn't to say that it's impossible, but that at this point a person cannot definitively state that it's true.
(h/t New York Times)