A research team from the United States and Australia reported these findings in the January 22 Journal of the American Medical Association.

Recognizing that genetic and environmental factors can influence a person’s decision to use marijuana or other drugs, the researchers studied a sample in which they could control for these factors while studying the effect of early marijuana use.

They studied 311 pairs of same-sex twins in which one twin in each pair began using marijuana before age 17 and the other did not. The twins came from the Australian Twin Register, a volunteer listing of twins born from 1964 to 1971.

The data are based on a single phone interview conducted between 1996 and 2000, when the mean age of the sample was 30.

"By studying twins who are discordant for cannabis use, we had the ideal way of controlling for genetic and family backgrounds," said lead author Michael Lynskey, Ph.D., a visiting professor of psychiatry at the Washington University School of Medicine in St. Louis and a senior research fellow at the Queensland Institute of Medical Research in Brisbane, Australia.

"The twins had the same parents, came from the same socioeconomic background, and, in the case of identical twins, shared the same genetic material," he told Psychiatric News.

The researchers found that among those who used marijuana before age 17, the odds of using other drugs increased. For instance, the odds of sedative use tripled, opioid use more than doubled, and hallucinogen use increased fivefold compared with twins who either never used marijuana or used the drug only after age 17.

In addition, early marijuana users had higher rates of abuse of and dependence on alcohol and other drugs than their twins.

Of the 311 twins who used marijuana before age 17, 148 (47.6 percent) abused or became dependent on any illicit drug compared with 102 (32.8 percent) of the 311 twins who did not use marijuana before age 17.

About 46 percent of early marijuana users later abused or became dependent on marijuana, compared with 32 percent of those who did not use marijuana before 17.

Early users also had higher rates of alcohol dependence (42.8 percent versus 29.6 percent) and abuse or dependence on cocaine and stimulants (12.5 percent vs. 4.5 percent).

Lynskey said that after controlling for genetic and environmental factors, he did not expect to find a link between early marijuana use and later drug use problems. "We were quite surprised," he said.

In addition, he acknowledged that although the researchers controlled for genetic and environmental factors to the best of their abilities to test the effects of early marijuana use, "nonshared environments" could not be accounted for and may have played a role in why the twins used marijuana at an early age or other drugs later in life.

For example, the parents may have treated one twin differently from the other, or each twin may have had his or her own group of friends outside school—factors that could potentially influence whether a person decides to use marijuana or other drugs later in life.

What are the factors that link early marijuana use to more serious drug problems down the road? Early experiences with marijuana are often pleasurable, the researchers posited, and so youngsters may be likely to replicate the pleasurable experience with more marijuana use or the use of other drugs.

They may also have had "safe" experiences—that is, their parents or law enforcement officials did not discover the youngster’s marijuana use. This "may reduce the perceived risk of, and therefore barriers to, the use of other drugs," according to the report.

In addition, when youngsters begin using marijuana at an early age, over time they may have increased exposure to drug dealers and other drugs.

This theory, Lynskey pointed out, "is often used to support sanctions against cannabis, but also has been used for state decriminalization of cannabis." He noted that government officials in the Netherlands, acting on the assumption that marijuana serves as a gateway to drug dealers and thus harder drugs, legalized marijuana so that people would have less contact with these other drugs in their pursuit of marijuana.

Although the association between early marijuana use and later drug use and abuse emerged in the study findings, Lynskey emphasized that "the majority of those who use cannabis at an early age did not go on to abuse or become dependent on other drugs."

Although Lynskey said he believes the findings from the twin study can be generalized to populations in the United States and other places in the world, he would like to see the study replicated elsewhere to "focus on the mechanisms underlying the association between early cannabis use and that of other drugs" and protective factors in early cannabis users who do not develop drug abuse or dependence.

An abstract of the study, "Escalation of Drug Use in Early-Onset Cannabis Users vs. Co-Twin Controls" is posted on the Web at http://jama.ama-assn.org/issues/v289n4/abs/joc21156.html. ▪

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