title: Escalation of drug use in early-onset cannabis users vs co-twin controls.
abstract: BACKGROUND
  Previous studies have reported that early initiation of cannabis (marijuana) use is a significant risk factor for other drug use and drug-related problems.
  
  
  OBJECTIVE
  To examine whether the association between early cannabis use and subsequent progression to use of other drugs and drug abuse/dependence persists after controlling for genetic and shared environmental influences.
  
  
  METHODS
  Cross-sectional survey conducted in 1996-2000 among an Australian national volunteer sample of 311 young adult (median age, 30 years) monozygotic and dizygotic same-sex twin pairs discordant for early cannabis use (before age 17 years).
  
  
  METHODS
  Self-reported subsequent nonmedical use of prescription sedatives, hallucinogens, cocaine/other stimulants, and opioids; abuse or dependence on these drugs (including cannabis abuse/dependence); and alcohol dependence.
  
  
  RESULTS
  Individuals who used cannabis by age 17 years had odds of other drug use, alcohol dependence, and drug abuse/dependence that were 2.1 to 5.2 times higher than those of their co-twin, who did not use cannabis before age 17 years. Controlling for known risk factors (early-onset alcohol or tobacco use, parental conflict/separation, childhood sexual abuse, conduct disorder, major depression, and social anxiety) had only negligible effects on these results. These associations did not differ significantly between monozygotic and dizygotic twins.
  
  
  CONCLUSIONS
  Associations between early cannabis use and later drug use and abuse/dependence cannot solely be explained by common predisposing genetic or shared environmental factors. The association may arise from the effects of the peer and social context within which cannabis is used and obtained. In particular, early access to and use of cannabis may reduce perceived barriers against the use of other illegal drugs and provide access to these drugs.
authors.affiliation: Queensland Institute of Medical Research, Brisbane, Queensland, Australia. mlynskey@matlock.wustl.edu
authors.forename: Michael T
authors.initials: MT
authors.lastname: Lynskey
authors.forename: Andrew C
authors.initials: AC
authors.lastname: Heath
authors.forename: Kathleen K
authors.initials: KK
authors.lastname: Bucholz
authors.forename: Wendy S
authors.initials: WS
authors.lastname: Slutske
authors.forename: Pamela A F
authors.initials: PA
authors.lastname: Madden
authors.forename: Elliot C
authors.initials: EC
authors.lastname: Nelson
authors.forename: Dixie J
authors.initials: DJ
authors.lastname: Statham
authors.forename: Nicholas G
authors.initials: NG
authors.lastname: Martin
mesh_terms: D000293:Adolescent; D000328:Adult; D017668:Age of Onset; D003430:Cross-Sectional Studies; D005260:Female; D006801:Humans; D016015:Logistic Models; D008297:Male; D002189:Marijuana Abuse / Q000453:epidemiology / Q000503:physiopathology*; D001523:Mental Disorders; D012307:Risk Factors; D012959:Socioeconomic Factors; D019966:Substance-Related Disorders / Q000453:epidemiology*; D014429:Twins, Dizygotic; D014430:Twins, Monozygotic