There are obviously two different questions here. One is the relation between cannabis and subsequent addiction to other substances, which is also known as the gateway theory, which has been studied and reported on for a number of years. It basically says that a lot of studies have shown that a vast majority of people will go on to use cocaine, heroin, or what are called hard drugs. When you look back at their story of substance use, they started by using cannabis, or quite often cannabis will be on the path of using other drugs. That has led to this idea that maybe cannabis was leading to other addictions that obviously as for all or most other outcomes what has been shown actually.... It's pretty much a similar story to psychosis. Indeed there is some data showing that cannabis may have some impact on how the brain will develop that could put someone at an increased risk of developing an addiction.
Obviously there are other much more important factors involved in developing addiction to other drugs.
The other thing we have to take into account goes back to what I was saying, that, in general, cannabis is not related to long-term harmful consequences. Most people or even most adolescents who use cannabis will not go on to use harder drugs. From epidemiological data, we know there's a minority of people who will go on to use other harder drugs, but still, cannabis is along the way. We know there is some neurobiological effect of early cannabis exposure that could increase in some ways the risk of developing other addictions.
As for psychosis, and again to put it briefly, what is known now is that cannabis per se is probably not itself a cause of schizophrenia or of long-term psychotic disorders. What we know now is that cannabis can act as a stressor or as a trigger for people who have a vulnerability with respect to psychosis; when exposed to cannabis, they will go on to develop schizophrenia, for example.