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Home > Publications > NIDA Notes > Vol. 18, No. 4 > Research Findings

Twins Study Links Early Marijuana Use to Increased Risk of Abuse or Dependence
Research Findings
Vol. 18, No. 4 (November 2003)

By Patrick Zickler, NIDA NOTES Staff Writer

Many genetic, biological, and environmental factors can influence whether and when an individual initiates drug abuse or develops drug dependence or addiction. One tool that helps scientists isolate and evaluate the effect of different factors is research on twins, who share many inherited biological traits and environmental influences. In a study of more than 300 pairs of same-sex twins, NIDA-supported investigators found that smoking marijuana before age 17 is linked to a greater likelihood of proceeding to serious problems with marijuana or other drugs.

Marijuana-Using Twin Teens More Likely To Use Drugs, Become Dependent
Marijuana-Using Twin Teens More Likely To Use Drugs, Become Dependent

Researchers investigated the drug use patterns of same sex twin pairs in which one twin first smoked marijuana before age 17 and the other either never smoked marijuana or first smoked the drug at age 17 or older. As adults (average age 30), those who had smoked before age 17 were more likely than their siblings to have used other illicit drugs and to develop symptoms of abuse or dependence on marijuana, cocaine or other stimulants, opioids, or sedatives.

"This finding underlines the significance of early drug initiation," says Dr. Wilson Compton, director of NIDA's Division of Epidemiology, Services and Prevention Research. "Identical twins had the same inherited biological characteristics, and fraternal twins shared half their genes. All the twins had common family influences and social environments. Even though they had so much in common, something influenced one twin to take drugs earlier than the other, and that difference had a profound impact on later experience with drugs."

The same-sex twin pairs grew up in the same households and attended the same schools. In each pair, one twin smoked marijuana before his or her 17th birthday and the other did not. "When we interviewed the twins as adults, the early users were more than twice as likely to have taken other illicit drugs. They also were from two to five times more likely to move on to abuse or dependence on alcohol, marijuana, stimulants, opioids, or sedatives," says Dr. Michael Lynskey, who conducted the study with colleagues at the Washington University School of Medicine in St. Louis, Missouri; the Queensland Institute of Medical Research in Brisbane, Australia; and the University of Missouri in Columbia.

The researchers asked both members of 2,765 twin pairs included in the Australian Twin Register if they had ever smoked marijuana and, if so, how old they were when they smoked it for the first time. The researchers identified 311 pairs of same-sex twins (average age 30) in which one twin first smoked marijuana before age 17 and the other twin had either never smoked the drug (77 pairs) or did so for the first time at age 17 or older (234 pairs). Of the 311 twin pairs, 136 (74 female, 62 male) were identical and 175 (84 female, 91 male) were fraternal. The interviews were conducted by phone in Australia and the data analyzed by scientists at Washington University and the University of Missouri.

The investigators defined "use" as drug taking on one or more occasions for a nonmedical reason. The researchers defined "abuse" and "dependence" according to criteria adapted from the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV). Abuse was understood to involve taking the drug in physically hazardous situations or circumstances that interfered with major obligations. According to the DSM-IV criteria, twins described as drug- or alcohol-dependent had two or more of the following symptoms: needing increasingly larger amounts to achieve drug effect, using for longer periods or more frequently than intended, and continuing to use despite associated emotional problems or recurrent desire to cut down use.

Overall, the researchers found, twins who smoked marijuana before age 17 were more than twice as likely as their sibling to use opioids, three times as likely to use sedatives, three times as likely to use cocaine or other stimulants, and nearly four times as likely to use hallucinogens. Those who smoked marijuana before age 17 also were from 1.6 to 6 times as likely to have reported abuse or dependence on alcohol or an illicit drug. Nonetheless, Dr. Lynskey points out, the majority (52 percent) of twins who smoked marijuana before age 17 did not go on to develop abuse or dependence. The increased odds of using other drugs or for developing abuse or dependence were not greater for identical than for fraternal twins, nor for males or females.

"While these study findings indicate that early marijuana use is associated with increased risk of progression to other illicit drug use and possibly to drug abuse or dependence, it is not possible to draw strong causal conclusions solely on the basis of these associations," Dr. Lynskey cautions. Additional research in other cultures, using a range of research designs, will be needed to determine the causes of the association, he says.

"Given that early initiation of marijuana smoking appears to be associated with increased risks," says Dr. Lynskey, "there is a need for greater physician awareness of those risks. Focused interventions also are needed to prevent escalation to use of other drugs among young people identified as being at risk."


Lynskey, M.T., et al. Escalation of drug use in early-onset cannabis users vs. co-twin controls. Journal of the American Medical Association 289(4):427-433, 2003.
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Volume 18, Number 4 (November 2003)

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