A developmental sequence of involvement in drugs is one of the best replicated findings in the epidemiology of drug use. Regular sequences and stages of progression in which the use of alcohol and cigarettes precedes the use of marijuana (cannabis), and, in turn, the use of marijuana precedes the use of other illicit drugs, has been observed in the United States as well as in other western societies.1 Very few individuals who have tried cocaine and heroin have not already used marijuana; the majority have previously used alcohol or tobacco. Such behavioral regularities are subsumed under the "gateway hypothesis." The gateway hypothesis implies 3 interrelated propositions about sequencing, association of initiation, and causation.1Sequencing implies that there is a fixed relationship between 2 substances, such that one substance is regularly initiated before the other. Association implies that initiation of one substance increases the likelihood of initiation of the second substance. Causation implies that use of the first substance actually causes use of the second substance. Causation, a controversial proposition, is the one most widely invoked in policy debates and is the proposition addressed in the article by Lynskey et al in this issue of THE JOURNAL.2
Kandel DB. Does Marijuana Use Cause the Use of Other Drugs? JAMA. 2003;289(4):482–483. doi:10.1001/jama.289.4.482
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