Marijuana, when used in strong concentrations such as skunk weed, more than once a week, has been shown to increase a teenager's chances of developing psychosis and schizophrenia like symptoms (I say schizophrenia like, because they usually fade away months after the weed is gone, unlike true schizophrenia). But, any chemical added to a developing brain bears the same risks. Alcohol can cause psychosis in certain people, atni-d's can make kids kill themselves. There have even been cases (I'm one of them) of birth control causing psychological problems in teens.