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Does marijuana use cause schizophrenia? (unclear, but unlikely)

Does Mar­i­jua­na Use Cause Schiz­o­phre­nia? (The New York Times):

… The con­cern is focused large­ly on the link between heavy usage and psy­chosis in young peo­ple. Doc­tors first sus­pect­ed a link some 70 years ago, and the evi­dence has only accu­mu­lat­ed since then. In a forth­com­ing book, “Tell Your Chil­dren,” Alex Beren­son, a for­mer Times reporter, argues that legal­iza­tion is putting a gen­er­a­tion at high­er risk of schiz­o­phre­nia and oth­er psy­chot­ic syn­dromes. Crit­ics, includ­ing lead­ing researchers, have called the argu­ment overblown, and unfaith­ful to the sci­ence

The debate cen­ters on the dis­tinc­tion between cor­re­la­tion and cau­sa­tion. Peo­ple with psy­chot­ic prob­lems often use cannabis reg­u­lar­ly; this is a sol­id cor­re­la­tion, backed by numer­ous stud­ies. But it is unclear which came first, the cannabis habit or the psy­choses. Chil­dren who lat­er devel­op schiz­o­phre­nia often seem to retreat into their own world, stalked peri­od­i­cal­ly by bizarre fears and fan­tasies well out­side the range of usu­al child­hood imag­i­na­tion, and well before they are exposed to cannabis. Those who go on to become reg­u­lar mar­i­jua­na users often use oth­er sub­stances as well, includ­ing alco­hol and cig­a­rettes, mak­ing it more dif­fi­cult for researchers to untan­gle cau­sa­tion…

The evi­dence so far indi­cates that one’s famil­ial risk for psy­chot­ic dis­or­ders out­weighs any added effect of cannabis use.”

The Study:

A con­trolled fam­i­ly study of cannabis users with and with­out psy­chosis (Schiz­o­phre­nia Research). From the abstract:

  • BACKGROUND: Cannabis is one of the most high­ly abused illic­it drugs in the world. Sev­er­al stud­ies sug­gest a link between ado­les­cent cannabis use and schiz­o­phre­nia. An under­stand­ing of this link would have sig­nif­i­cant impli­ca­tions for legal­iza­tion of cannabis and its med­i­c­i­nal val­ue. The present study aims to deter­mine whether famil­ial mor­bid risk for schiz­o­phre­nia is the cru­cial fac­tor that under­lies the asso­ci­a­tion of ado­les­cent cannabis use with the devel­op­ment of schiz­o­phre­nia.
  • RESULTS: There was an increased mor­bid risk for schiz­o­phre­nia in rel­a­tives of the cannabis using and non-using patient sam­ples com­pared with their respec­tive non-psy­chot­ic con­trol sam­ples (p=.002, p<.001 respec­tive­ly). There was no sig­nif­i­cant dif­fer­ence in mor­bid risk for schiz­o­phre­nia between rel­a­tives of the patients who use or do not use cannabis (p=.43).
  • CONCLUSIONS: The results of the cur­rent study sug­gest that hav­ing an increased famil­ial mor­bid risk for schiz­o­phre­nia may be the under­ly­ing basis for schiz­o­phre­nia in cannabis users and not cannabis use by itself.

The Study in Context:

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