In a large study of Australian twins, the researchers found that those who used marijuana before age 17 were two to five times more likely to use other drugs or to develop alcohol or drug abuse or dependence. The study appears in the Jan. 22 issue of the Jo urnal of the American Medical Association.
"There is a fairly long history of research showing that early cannabis (marijuana) use is associated with increased risks for later use of so-called 'hard drugs,' but that research is based on the fact that most heroin and cocaine users report first having used cannabis," says lead author Michael T. Lynskey, Ph.D., a visiting assistant professor of psychiatry at Washington University School of Medicine in St. Louis and senior research fellow at the Queensland Institute of Medical Research in Brisbane, Australia.
Lynskey says past studies have not been able to adequately control for familial factors -- such as genetics, environment and family background -- that may predispose people both to early marijuana use and to subsequent use of illicit drugs. In this study, Lynskey and colleagues from Washington University and the Queensland Institute of Medical Research studied same-sex twins from Australia: some identical, some fraternal. In 311 pairs of twins, one twin began using marijuana before the age of 17 and the other did not.
"By studying twins, we were able to compare pairs of individuals of the same age, same family background and -- in the case of identical twins -- individuals with exactly the same genes," Lynskey explains. "But these twins differed in one important respect: One had chosen to begin using cannabis before 17, but the other had not."
By the time these twins were interviewed in their late 20s and early 30s, the early marijuana users had developed higher rates of problems with alcohol and other drugs. Some 46 percent reported that they later abused or became dependent upon marijuana, and 43 percent had become alcohol dependent.
The early marijuana users also used other drugs at higher rates, including cocaine and other stimulants (48 percent) heroin and other opioids (14 percent) and hallucinogens (35 percent).
"Controlling for other known risk factors for drug use and drug use problems, these rates were between 1.8 and 5.2 times higher than the rates we observed in the co-twins who did not begin cannabis use before 17," Lynskey says. Results were similar when comparisons were limited to identical twin pairs.
"We actually were expecting that by using twins and controlling for genetic and familial effects, we'd find the association between early use and later abuse would disappear," Lynskey says. "But this study demonstrates that there is more to the relationship than we previously thought."
Study leader Andrew Heath, D. Phil, Olin Professor of Psychiatry and director of the Missouri Alcoholism Research Center at Washington University School of Medicine, agrees.
"I think one important thing to say to the parents of a 16-year-old using marijuana is that the majority of kids who use cannabis do not go on to experience problems with drugs or alcohol, but it's important that we, as parents and as a society, recognize that there is an increased risk," Heath says.
Heath believes that marijuana use by children and their parents has been so frequent that societies in the United States and Australia may downplay the importance of early marijuana use as a potential risk factor for later problems.
Lynskey agrees, but he believes it also is important to emphasize that simply using marijuana prior to age 17 does not guarantee problems.
It is not clear how early use of marijuana might be related to later substance problems. Although this study suggests that genetic and environmental factors alone cannot explain the risk, Lynskey and Heath say those factors clearly play some role. They believe it also is likely that people who use marijuana at a young age may be more likely to be involved in a lifestyle that will put them at increased risk for a whole range of problems.
"I think it often is implicitly assumed that the association between cannabis and other drugs is somehow pharmacological, that using cannabis changes your brain or makes you crave other drugs," Lynskey says. "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 behaviors."
Lynskey MT, Heath AC, Bucholz KK, Slutske WS, Madden PAF, Nelson EC, Statham DJ, Martin NG. Escalation of Drug Use in Early-Onset Cannabis Users vs. Co-twin Controls. JAMA, vol. 289:4, pp. 427-433, Jan. 22, 2003, with accompanying editorial on pp. 482-483.
This research was supported by grants from the National Institute on Alcohol Abuse and Alcoholism(NIAAA) and the National Institute on Drug Abuse (NIDA) of the National Institutes of Health, and from the Australian National Health and Medical Research Council.