A study of Australian twins appears to bolster the theory that marijuana can lead to harder drugs.

The researchers found 311 sets of same-sex twins, including 136 sets of identical twins, in which just one twin had smoked marijuana before age 17. Early marijuana smokers were found to be up to five times more likely to move to harder drugs than were their twins. They were about twice as likely to use opiates like heroin and five times as likely to use hallucinogens like LSD.

Other studies reached conflicting conclusions.

The new study, published today in The Journal of the American Medical Association, is unlikely to still the debate. In an accompanying editorial, Dr. Denise B. Kandel of the psychiatry department at Columbia University said the study did not explain "whether or not a true causal link exists" between marijuana and hard drugs.

"An argument can be made," Dr. Kandel wrote, "that even identical twins do not share the same environment during adolescence."

The study does not say 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 the lead researcher, Dr. Michael T. Lynskey of the Queensland Institute of Medical Research in Australia. "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."

Dr. Lynskey and his colleagues acknowledged that the study had limitations, including relying on participants' reporting their experiences. The participants, whose average age was 30, were asked about their drug use as teenagers. 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.