According to background information in the article, in 1999 there were 220,000 marijuana-related admissions to publicly funded substance abuse treatment programs in the U.S. This represented 14 percent of all such treatment admissions, with admissions occurring primarily among youth: Roughly one third of all marijuana-related admissions were for youths aged 12 to 17 years old.

The hypothesis that early marijuana use may increase the risk for later use of other drugs has been a major cause for concern. This idea, known as the gateway hypothesis, asserts that there is an order in the progression of drug use, with marijuana use preceding the use of harder drugs such as cocaine and heroin.

Michael T. Lynskey, Ph.D., and his Australian colleagues studied marijuana use and its effects on later drug use among 311 sets of same-sex twins (average age at the start of the study, 30 years old) where only one twin used marijuana before age 17 years. The study was conducted among a national volunteer sample between 1996 and 2000.

The researchers found that individuals who used marijuana before age 17 years had a likelihood of other drug use, alcohol dependence, and drug abuse/dependence that were 2.1 to 5.2 times higher than the risk of twins who did not use marijuana before 17 years of age. Adjustments to the data for known risk factors for drug abuse such as early use of alcohol and tobacco, parental conflicts or separation, childhood sexual abuse, conduct disorder, major depression, and social anxiety had little effect on the results. The associations found between marijuana use and other drug and alcohol use did not differ between monozygotic and dizygotic twins.

The researchers wrote, "The results of our co-twin analysis indicated that early initiation of cannabis use was associated with significantly increased risks for other drug use and abuse/dependence and were consistent with early cannabis use having a causal role as a risk factor for other drug use and for any drug abuse or dependence."

"Associations between early cannabis [marijuana] 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 the use of cannabis may reduce perceived barriers against the use of other illegal drugs and provide access to these drugs," the authors stated.

In an accompanying editorial, Denise B Kandel, Ph.D., wrote "... Lynskey et al find that early marijuana use by itself, even after control for other covariates, increases significantly the use of other illicit drugs. As the authors emphasize, the strength of the twin design is that twins are assumed to share the same environment and family experiences and monozygotic pairs share the same genetic risk."

Dr Kandel noted, "Whether or not a true causal link exists between the use of marijuana and other drugs, the association between the 2 has been well established. It is important, however, to appreciate that the progression is not inevitable. Not all those who try marijuana will subsequently use cocaine or become heroin addicts. For policy makers, the gateway hypothesis raises 2 issues depending on whether the population of interest has or has not yet used marijuana. For a population of nonusers, the issue is: Will preventing the use of marijuana prevent the use of other illicit drugs? Hopefully, it will, for prevention efforts will presumably affect the underlying risk and protective factors related to the onset of marijuana use, whether or not these factors are shared with the onset of the use of other illicit drugs. For youths who have already used marijuana, the issue is: Can and should intervention programs be developed to target this group at very high risk for progressing to other substances? It appears so. A marijuana user is at risk for using other illicit drugs, and measures to prevent subsequent use of these drugs are warranted."