One Tokes, The Other Doesn't

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Parents and school principals are always trying to scare kids with the message that smoking pot will lead to harder drugs. Well, a study in the Journal of the American Medical Association suggests that the grownups may be on to something. By tracking 311 pairs of Australian twins (both fraternal and identical) in which one twin used marijuana before age 17 and the other did not, researchers have been able to show that early pot users are as much as five times more likely to use or abuse cocaine, heroin, hallucinogens, sedatives or alcohol.

This isn't the first study to call pot a gateway drug. What makes this one so persuasive is that it factors in such things as economic background, family upbringing and even, in the case of identical twins, genetic makeup. "We actually were expecting that by using twins, we'd find that the association between early use and later abuse would disappear," says Michael Lynskey, a visiting psychiatry professor at Washington University in St. Louis, Mo., and the study's lead author.

Does marijuana use trigger subtle biochemical changes that encourage drug-taking behavior? Or does cannabis give kids an entree into social circles where stronger drugs are used? The study doesn't have all the answers, but it does offer kids another reason to heed their elders.