I think the cannibus seed bank owner made a good point about how a distinction needs to be made between psychotic symptom, psychotic episode, psychosis and schizophrenia.
There have been longitudinal studies (studies where they have followed a persons develop for a number of years) that has shown that heavy cannibus users are about 2.5 or 3 times more likely to develop schizophrenia than non-users (I think the study was called the Swedish Cohort study). The take home messages of these types of studies is that whilst cannibas may contribute to a person developing schizophrenia, it does not cause schizophrenia on its own, there were other factors as well as cannibas that contributed to that person developing schizophrenia. In other words, there is a casual link between cannibas and schizophrenia.
That said, cannibas can cause symptoms or mental states that are very similar to psychotic symptoms or mental states. For example, I'm sure there are plenty of people who use bluelight who have become paranoid for no reason or have had some sort of delusions (generally negative) when smoking pot. these are similar to what a person who is having a psychotic episode experiences. However, when someone smokes pot these psychotic symptoms will disappear. Adding to this there has been no comprehensive research as to whether there are any close neurobiological similarities between delusions on cannibas and delusions suffered during psychosis.
There are several take home messages:
1. Not enough is known about the causes of schizophrenia for any one to infer that there is a direct cause, so far all we know is that several casual factors (like pot) may lead to succeptable individuals developing schizophrenia
2. Young minds are still developing up until about the age of 21. Adolescence is a time when alot of experimentation is done with many things, inlcuding drugs. It is also a time when a person is developing a sense of identity and many stressful changes are occuring. It is also the most likely time that someone is to develop a mental illness like schizophrenia. Young people would do well not to use drugs on a regular basis at this stage, just adds to more casual factors that can lead to the development of schizophrenia.
3. Cannibas psychosis has been shown to occur in some heavy users of cannabis, a psychosis is generally an episode that can last anywhere from a couple of hours to generally at most a month, if use is discontinued. However, generally this will only happen to individuals who are succeptable to psychotic episodes - ie. the episode may have happened later on in this persons life, but the cannibas triggered it early, or the heavy smoking was the hair that broke the camels back.
Pretty much this article was a skimming of evidence suggesting a link between psychosis/schizophrenia and cannabis but didn't elaborate at all the complexities of the argument. It was sensational journalism, dumbed down so that your averaage howard, simpson and his donkey loving, aussie battler could have something to grumble about over his VB and winnie blues. There was some truth in it, but as said before this will not happen to vast majority of people and in my opinion probably won't happen at all if people keep their cannabis use to a low level.