title: A twin study of early cannabis use and subsequent use and abuse/dependence of other illicit drugs.
abstract: BACKGROUND
  Cannabis use is strongly associated with the use and abuse/dependence of other illicit drugs. Gateway and common liabilities models have been employed to explain this relationship. We sought to examine this association using a combination of the discordant twin design and modeling methods.
  
  
  METHODS
  We assess the relationship between early cannabis use and the subsequent use and abuse/ dependence of other illicit drugs in a population-based sample of male and female twin pairs using four analyses: (i) analysis of the association between early cannabis use and other illicit drug use and abuse/dependence in the entire sample of twins, (ii) assessment of the influence of early cannabis use in twin 1 on twin 2's use or abuse/dependence of other illicit drugs, (iii) use of twin pairs discordant for early cannabis use in a discordant twin design and (iv) a model-fitting procedure.
  
  
  RESULTS
  We found: (i) a strong association between early cannabis use and use and abuse/dependence of other illicit drugs in the sample, (ii) twin 1's early cannabis use is significantly associated with the twin 2's other illicit drug use, (iii) the role of correlated genetic factors with some evidence for a causal influence, and (iv) the correlated liabilities model fits the data well.
  
  
  CONCLUSIONS
  Early cannabis use is strongly associated with other illicit drug use and abuse/dependence. The relationship arises largely due to correlated genetic and environmental influences with persisting evidence for some causal influences.
authors.affiliation: Virginia Commonwealth University, Virginia Institute for Psychiatric and Behavioral Genetics, Department of Human Genetics, Richmond, VA 23298-0003, USA. aagrawa@hsc.vcu.edu
authors.forename: Arpana
authors.initials: A
authors.lastname: Agrawal
authors.forename: Michael C
authors.initials: MC
authors.lastname: Neale
authors.forename: Carol A
authors.initials: CA
authors.lastname: Prescott
authors.forename: Kenneth S
authors.initials: KS
authors.lastname: Kendler
mesh_terms: D000328:Adult; D015984:Causality; D004200:Diseases in Twins / Q000453:epidemiology / Q000235:genetics* / Q000523:psychology; D005260:Female; D005838:Genotype; D006801:Humans; D013287:Illicit Drugs; D008297:Male; D002189:Marijuana Abuse / Q000453:epidemiology / Q000235:genetics* / Q000523:psychology; D008875:Middle Aged; D012306:Risk; D012931:Social Environment; D013223:Statistics as Topic; D019966:Substance-Related Disorders / Q000453:epidemiology / Q000235:genetics* / Q000523:psychology; D014429:Twins, Dizygotic / Q000235:genetics / Q000523:psychology; D014430:Twins, Monozygotic / Q000235:genetics / Q000523:psychology; D014768:Virginia
chemical_list: D013287:Illicit Drugs