The link between weed and psychosis

While smoking might attribute to heightened cognition or even a cancer cure, it might also cause other issues like early Alzheimer's or depression....

This article "The Link Between Marijuana and Schizophrenia" shares about how studies show that cannabis smokers are more likely to develop schizophrenia than non-smokers. I suppose it depends on how it is used wisely, since shamans are known to use cannabis for spiritual and medicinal purposes due to its hallucinatory and medicinal effects respectively. 

According to this article, younger people are more susceptible to the psychotic effects of cannabis. 

http://www.rcpsych.ac.uk/expertadvice/problems/alcoholanddrugs/cannabis.aspx

"Why should teenagers be particularly vulnerable to the use of cannabis? No one knows for certain, but it may be something to do with brain development. The brain is still developing in the teenage years – up to the age of around 20, in fact. A massive process of ‘neural pruning’ is going on. This is rather like streamlining a tangled jumble of circuits so they can work more effectively. Any experience, or substance, that affects this process has the potential to produce long-term psychological effects."

Cannabis then need to be reserved for medicinal use instead of recreational use, since cannabis is found to be effective cancer cure. Also, some studies suggest that whether people develop psychosis may depend on their genetic predisposition to develop mental illness.

http://en.m.wikipedia.org/wiki/Long-term_effects_of_cannabis

"Both advocates and opponents of the drug are able to call upon numerous scientific studies supporting their respective positions. For instance, while cannabis has been implicated in the development of various mental disorders in some studies^[which?], these studies differ widely as to whether cannabis use is the cause of the mental problems displayed in heavy users, whether the mental problems are exacerbated by cannabis use, or whether both the cannabis use and the mental problems are the effects of some other cause. Positive effects of long-term cannabis use have also been noted.^[1]"

^{So, it boils down to knowing when and how to use cannabis appropriately. It may not work well for everyone, and is best used only for a temporal period of time, and for medicinal purposes where necessary.}