This study investigates the relationship between cannabis use in adolescence and the risk of developing adult schizophreniform disorder, while controlling for childhood psychotic symptoms. The Dunedin Multidisciplinary Health and Development Study, a longitudinal cohort study of 1037 individuals born in Dunedin, New Zealand, in 1972-1973, was used. The study followed 759 participants with complete data on adult psychiatric outcomes, adolescent drug use, and childhood psychotic symptoms. The sample was divided into three groups based on cannabis use at ages 15 and 18: controls (never or once/twice users), early users (three or more times at age 18), and late users (three or more times at age 15). The results showed that cannabis users by age 15 and 18 had significantly more schizophrenia symptoms at age 26 compared to controls, even after controlling for childhood psychotic symptoms. Logistic regression analyses indicated that early cannabis users (by age 15) were four times more likely to be diagnosed with schizophreniform disorder at age 26, although this risk was reduced after controlling for childhood psychotic symptoms. The study concludes that adolescent cannabis use increases the likelihood of experiencing schizophrenia symptoms in adulthood, with early use being more risky than later use. The findings suggest that cannabis use among psychologically vulnerable adolescents should be strongly discouraged, and policy makers should focus on delaying the onset of cannabis use.This study investigates the relationship between cannabis use in adolescence and the risk of developing adult schizophreniform disorder, while controlling for childhood psychotic symptoms. The Dunedin Multidisciplinary Health and Development Study, a longitudinal cohort study of 1037 individuals born in Dunedin, New Zealand, in 1972-1973, was used. The study followed 759 participants with complete data on adult psychiatric outcomes, adolescent drug use, and childhood psychotic symptoms. The sample was divided into three groups based on cannabis use at ages 15 and 18: controls (never or once/twice users), early users (three or more times at age 18), and late users (three or more times at age 15). The results showed that cannabis users by age 15 and 18 had significantly more schizophrenia symptoms at age 26 compared to controls, even after controlling for childhood psychotic symptoms. Logistic regression analyses indicated that early cannabis users (by age 15) were four times more likely to be diagnosed with schizophreniform disorder at age 26, although this risk was reduced after controlling for childhood psychotic symptoms. The study concludes that adolescent cannabis use increases the likelihood of experiencing schizophrenia symptoms in adulthood, with early use being more risky than later use. The findings suggest that cannabis use among psychologically vulnerable adolescents should be strongly discouraged, and policy makers should focus on delaying the onset of cannabis use.