Table 4.

Adolescent cannabis use and schizophrenia.

Authors and year	Location	Type of study	N	Diagnosis	Finding
Andreasson et al. (1987)	Sweden	Historical cohort study of Swedish military conscripts from 1969	246	ICD-8 Diagnosis of schizophrenia and other psychoses determined by psychiatric interview	High cannabis consumption (use on more than 50 occasions) at conscription (age 18) had a six-fold increase in risk of schizophrenia after 15-year follow-up compared with nonusers
Arseneault et al. (2002)	Dunedin, New Zealand	Prospective longitudinal birth cohort study	759	DSM-IV Diagnoses of schizophrenia and depression and diagnoses of schizophreniform disorder and depression determined with a standardized interview	Subjects who used cannabis by age 15 were four times more likely to have a diagnosis of schizophreniform disorder at age 26 than controls. After psychotic symptoms at age 11 were controlled for, risk for adult schizophreniform disorder remained higher among those who used cannabis at age 15; however, this risk was no longer significant
Zammit et al. (2002)	Sweden	Retrospective cohort study of Swedish conscripts from 1969	362 cases	ICD-8 diagnoses of schizophrenia and other psychoses determined by record linkage	Use of cannabis in adolescence was associated with increase in risk of developing schizophrenia in a dose dependent fashion both for subjects who had ever used cannabis and for subjects who had used only cannabis and no other drugs. Subjects who had ever used cannabis had only a 1.2-fold increase in risk, while subjects who used cannabis > 50 times had a 6.7-fold increase in risk
Fergusson et al. (2003)	Christchurch, New Zealand	Prospective longitudinal birth cohort study	1011 subjects assessed at both age 18 and 21	Psychotic symptoms assessed in an interview using the Symptom Checklist 90 (SCL-90)	Cannabis dependence disorder at age 18 associated with a two-fold increased risk of psychotic symptoms at age 21, even after adjustment for prior psychotic symptoms
Caspi et al. (2005)	Dunedin, New Zealand	Prospective longitudinal birth cohort study	36 cases	DSM-IV diagnosis of schizophreniform disorder determined with a standardized interview	A functional variation in the catechol-O-methyltransferase (COMT) gene increased the effect of adolescent cannabis use on risk of adult psychosis
Henquet et al. (2005)	Germany	Prospective data analysis from a population based sample	2437 total subjects	Symptoms of psychosis determined by psychiatric interview	Cannabis use at baseline increased the cumulative incidence of psychotic symptoms by 1.7-fold after adjustment for age, sex, socioeconomic status, urbanicity, childhood trauma, predisposition for psychosis at baseline, and use of other drugs, tobacco, and alcohol. The effect of cannabis use was stronger in those with any predisposition for psychosis at baseline than in those without. The risk difference in the “predisposition” group was significantly greater than the risk difference in the “no predisposition” group. Predisposition for psychosis at baseline did not significantly predict cannabis use four years later