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

The study appears in Wednesday'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 the non-early marijuana users. Hallucinogens were the second most common, used by 35 percent of the early marijuana twins versus 18 percent of the others.