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Previous work has highlighted the adverse consequences of early-onset cannabis use. However, little is known about the predictors and effects of early-onset amphetamine use. We set out to examine these issues using a representative cohort of young people followed-up over 11 years in Victoria, Australia.A stratified, random sample of 1943 adolescents was recruited from secondary schools across Victoria at age 14–15 years. This cohort was interviewed on eight occasions until the age of 24–25 years (78% follow-up at that age). Cross-sectional associations were assessed using logistic regression with allowance for repeated measures. Both proportional hazards models and logistic regression models were used to assess prospective associations.Approximately 7% of the sample had used amphetamines by the age of 17 years. Amphetamine use by this age was associated with poorer mental health and other drug use. The incidence of amphetamine use during the teenage years was predicted by heavier drug use and by mental health problems. By young adulthood (age 24–25 years), adolescent amphetamine users were more likely to meet criteria for dependence upon a range of drugs, to have greater psychological morbidity and to have some limitations in educational attainment. Most of these associations were not sustained after adjustment for early-onset cannabis use.Young people in Australia who begin amphetamine use by age 17 years are at increased risk for a range of mental health, substance use and psychosocial problems in young adulthood. However, these problems are largely accounted for by their even earlier-onset cannabis use.