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Lynskey MT, Heath AC, Bucholz KK, et al. Escalation of Drug Use in Early-Onset Cannabis Users vs Co-twin Controls. JAMA. 2003;289(4):427–433. doi:10.1001/jama.289.4.427
Author Affiliations: Queensland Institute of Medical Research, Brisbane, Queensland, Australia (Drs Lynskey and Martin and Ms Statham); Missouri Alcoholism Research Center and Department of Psychiatry, Washington University School of Medicine, St Louis (Drs Lynskey, Heath, Bucholz, Madden, and Nelson); and Missouri Alcoholism Research Center and Department of Psychology, University of Missouri, Columbia (Dr Slutske).
Context 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.
Design 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
Main Outcome Measures 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.
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