Some contend that marijuana has become a growing cause of mental illness. In truth, this has not been proven and it is clear that not all people who use marijuana will become mentally ill. There is evidence that supports the idea that marijuana use in adolescents is tied to an increased likelihood of developing depression or psychotic disorders, particularly schizophrenia and bipolar disorder. Use by those with congenital predispositions or certain genes is also tied to development of these disorders; marijuana can exacerbate the disorders’ symptoms and possibly bring them out sooner. Frequent marijuana use among teenagers is believed to be especially problematic because their brains are not yet fully developed; it is also linked to poor school performance. Whether marijuana use contributes to mental illness or not, recent studies have shown that marijuana use in those with mental illness can exacerbate the problem especially by derailing treatment efforts.
For more detailed reading:
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3221171/
http://www.camh.ca/en/hospital/about_camh/newsroom/news_releases_media_advisories_and_backgrounders/current_year/Pages/CAMH-study-shows-mental-illness-associated-with-heavy-cannabis-use-.aspx