There is some evidence (links below) that when people already disposed to certain personality, bi-polar, or schizophrenia disorders use cannabis regularly, they increase their chances of imbalance leading to potentially dramatic changes to their behavior, habits, and personality. The changes are less irreversible than the Phinneas Gage case though. Usually, abstaining from drugs with neurochemical ingredients and regular therapy can return a person's balance:
There is some evidence (links below) that when people already disposed to certain personality, bi-polar, or schizophrenia disorders use cannabis regularly, they increase their chances of imbalance leading to potentially dramatic changes to their behavior, habits, and personality. The changes are less irreversible than the Phinneas Gage case though. Usually, abstaining from drugs with neurochemical ingredients and regular therapy can return a person's balance:
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
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1124674/
http://bjp.rcpsych.org/content/178/2/116.short
http://linkinghub.elsevier.com/retrieve/pii/S0140673687926201?via=sd