A 38-year study of more than 1,000 New Zealanders found that adolescents who used marijuana at least four days per week lost an average of eight IQ points between the ages of 13 and 38.43 Those people who began heavy smoking of marijuana only in adulthood did not suffer a loss of IQ. Loss of IQ was not reversible once marijuana use was ceased.43
Some marijuana users can suffer from cannabis psychosis when they take large doses over a period of time, with symptoms characteristic of paranoid schizophrenia.44 A recent study found that marijuana use significantly increased the risk of developing mental health problems among those young people who possessed a genetic high risk for schizophrenia (familial risk factors).45 Among cannabis users who developed cannabis-induced psychosis, 44.5% developed schizophrenia-spectrum disorders, with about half of those being diagnosed more than a year after seeking treatment for their cannabis-induced psychosis.46 A study using a sample of 880 adolescents in Melbourne, Australia found that lifetime cannabis use and the frequency of cannabis use in the last year were associated with psychotic-like experiences (primarily the experience of auditory and visual hallucinations).47 Another study examined the association between cannabis use and psychosis in 3,800 participants involving a subset analysis of 228 sibling pairs over a 21-year period of time. The results showed that early use of marijuana was associated with psychosis-related outcomes in young adults.48
Another study examined the associations between cannabis use and the development of mood and anxiety disorders.49 The study found no association between cannabis used and the development of anxiety disorders, although there was a significant correlation with the development of depression and bipolar disorder.49 Another study found an association between cannabis use and the development of panic attacks.50
A preliminary DTI study showing no brain structural change associated with adolescent cannabis useThe Center for Advanced Brain Imaging, The Nathan S. Kline In stitute for Psychiatric Research , Orangeburg, New York, USA and 2 The Department of Psychiatry, New York University School of Medicine, New York, New York, U