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Is Marijuana a Gateway Drug?

Early initiation of marijuana use was associated with later drug and alcohol abuse.

Many policymakers and theorists believe that marijuana use leads to use of harder drugs. If this supposition is true, efforts to prevent marijuana use and to intervene with marijuana users are crucial. To find evidence to support or refute marijuana's role as a gateway drug, researchers surveyed 311 Australian monozygotic or dizygotic same-sex twins who were discordant for early marijuana use.

Most twins who reported use of other illicit drugs had used marijuana first. Early users of marijuana were more likely than late- and never-users to use other illicit drugs and to meet diagnostic criteria for illicit drug abuse or dependence or for alcohol dependence. In adjusted analyses that considered family, social, and individual factors (including psychiatric disorders, early tobacco use, and early alcohol use), early marijuana use remained significantly associated with higher odds of: sedative, hallucinogen, cocaine/stimulant, or opioid use; marijuana, cocaine/stimulant, or opioid abuse or dependence; any illicit drug abuse or dependence; and alcohol dependence.

Comment: So, does early marijuana use cause later illicit drug use? If the researchers effectively removed genetics and environment from the equation in this twin study, then the answer is yes. But, an editorialist questions whether even identical twins really share the same environment. In any case, regardless of whether the relation is causal, early marijuana use is associated with later drug use. Interventions aimed at risk factors for illicit drug use should target high-risk youth, including early users of marijuana.

— Richard Saitz, MD, MPH, FACP

Published in Journal Watch General Medicine February 18, 2003

Citation(s):

Lynskey MT et al. Escalation of drug use in early-onset cannabis users vs co-twin controls. JAMA 2003 Jan 22/29; 289:427-33.

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• Medline abstract (Free)

Kandel DB. Does marijuana use cause the use of other drugs? JAMA 2003 Jan 22/29; 289:482-3.

• Original article (Subscription may be required)
• Medline abstract (Free)