Marijuana Use Leads to Later Drug Abuse

Early Smoking Influences Later Drug, Alcohol Problems

Friends passing joint
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One of the controversies surrounding the marijuana legalization movement revolves around the question of whether marijuana is a gateway drug or not. If smoking weed does in fact lead to the use of other drugs, then legalizing the recreational use of marijuana may not be such a good idea after all.

Unfortunately, there is little scientific research that definitely resolves the gateway drug debate. There have been studies that returned results on both sides of the question.

One study, however, suggested that the age when someone begins to use marijuana is a key factor in whether they go on to problems with alcohol and other drugs later in life.

In a study of 311 pairs of same-gender twins, researchers found that those who began marijuana use before age 17 were 2.1 to 5.2 times more likely to use other drugs or to develop alcohol or drug abuse or dependence compared to their twin who did not use marijuana before 17.

Using Marijuana Before Age 17

"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 said 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 the Australian study, the researchers examined same-sex twins - some identical, some fraternal. In each of the 311 pairs of twins, one twin began using marijuana before the age of 17 and the other did not.

Higher Rates of Alcohol, Drug Problems

"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."

When these 311 sets of 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 used other drugs at higher rates, including cocaine and other stimulants (48 percent) heroin and other opioids (14 percent) and hallucinogens (35 percent).

Known Risk Factors for Drug Use

"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.

Other risk factors for alcohol and other drug abuse the researchers controlled for included early-onset alcohol or tobacco use, parental conflict/separation, childhood sexual abuse, conduct disorder, major depression, and social anxiety.

"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."

Recognizing the Increased Risk

The lead investigator Andrew Heath, D. Phil, Olin Professor of Psychiatry and director of the Missouri Alcoholism Research Center at Washington University School of Medicine, was also surprised.

"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.

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.

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