Stimulant use and symptoms of abuse/dependence: epidemiology and associations with cannabis use--a twin study.

BACKGROUND This paper explores the magnitude of--and extent of overlap between--additive genetic, shared environmental and non-shared environmental influences on lifetime stimulant use and on stimulant abuse/dependence symptoms; the associations between stimulant use and cannabis use and the extent to which these associations can be attributed to common or correlated genetic and environmental influences. METHODS Self-report data on lifetime stimulant use, abuse/dependence symptoms and corresponding measures of cannabis involvement were collected from a sample of 6265 male and female Australian twins born 1964-1971. RESULTS Approximately one in five study participants reported lifetime stimulant use while 5% reported experiencing at least one symptom of abuse/dependence. Multivariate genetic model fitting indicated moderate genetic influences on stimulant use (40%) and symptoms (65%) while there was no evidence of sex differences in the magnitude of these influences. Despite some overlap in genetic influences on these measures, approximately 60% of the genetic variance in symptoms was specific to this phenotype. There were also strong genetic and shared environmental correlations between the factors associated with stimulant use and those associated with cannabis use. CONCLUSIONS There were moderate genetic influences on stimulant use and stimulant abuse/dependence with moderate overlap between the genetic factors associated with these outcomes. Additionally, there were strong associations between measures of lifetime stimulant use and analogous measures of cannabis use which, importantly, could be largely attributed to shared familial risk factors predisposing to both stimulant and cannabis use.