STUDY ON TWINS SUPPORTS VIEW OF MARIJUANA AS A GATEWAY DRUG

A study of Australian twins and marijuana bolsters the fiercely
debated ''gateway theory'' that pot can lead to harder drugs.

The researchers located 311 sets of same-sex twins in which only one
twin had smoked marijuana before age 17. Early marijuana smokers were
found to be up to five times more likely than their twins to move on
to harder drugs.

They were about twice as likely to use opiates, which include heroin,
and five times more likely to use hallucinogens, which include LSD.

Earlier studies on whether marijuana is a gateway drug reached
conflicting conclusions. The impasse has complicated the debate over
medical marijuana and decriminalization of pot.

Because the study involved twins, the findings would suggest that
genetics plays a subordinate role in drug use.

The study appears in today's Journal of the American Medical
Association and was funded in part by the National Institutes of Health.

It does not answer how marijuana, or cannabis, might lead to harder
drugs.

''It is often implicitly assumed that using cannabis changes your
brain or makes you crave other drugs,'' said lead researcher Michael
Lynskey, ''but there are a number of other potential mechanisms,
including access to drugs, willingness to break the law, and
likelihood of engaging in risk-taking behavior.''

Lynskey is a senior research fellow at Queensland Institute of Medical
Research in Brisbane and a visiting assistant psychiatry professor at
Washington University in St. Louis, where some of the research was
done.

Lynskey and colleagues acknowledged the study has several limitations,
including relying on participants' reporting of their own
experiences.

In an accompanying editorial, Denise Kandel of Columbia University's
psychiatry department said the study does not explain ''whether or not
a true causal link exists'' between marijuana and hard drugs.

''An argument can be made that even identical twins do not share the
same environment during adolescence,'' she said.

Study participants were age 30 on average when they were asked about
their teenage drug use. They included 136 sets of identical twins, who
share the same genetic makeup.

About 46 percent of the early marijuana users reported that they later
abused or became dependent on marijuana, and 43 percent had become
dependent on alcohol.

Cocaine and other stimulants were the most commonly used harder drugs,
tried by 48 percent of the early marijuana users, compared with 26
percent of those who didn't try marijuana early.

Hallucinogens were the second most common, used by 35 percent of the
early marijuana twins versus 18 percent of the others.