An article here on research regarding the link between cannabis use and psychosis. It is from a group of scientists working in UK who say that cannabis use can lead to psychosis in certain people. the risk may be greater if use begins early, during adolescence. It may not be a risk to other people, but it needs to be taken seriously.
More research is needed to clarify strength and vulnerability factors in individuals, but that there is a link for some, and a loss of life potential, is already clear.
[This study is from the Guardian and reflects UK statistics, but recognizes trends elsewhere. Far more teenagers seek treatment in the UK for cannabis use than for any other drug including alcohol.]