The Gateway Drug: Your Genes?

The question of why some kids start using alcohol, cigarettes, marijuana, and other drugs at a young age remains a source of controversy. How much of a role do genes play? The environment -- peers, parents, educational efforts? What about the "gateway theory," the idea that one drug -- marijuana is the most likely to be blamed -- leads to use of others?

A new study of twins recently published online by the journal Drug and Alcohol Dependence suggests that genes may play a large role, but to some degree every drug is a gateway drug.

By studying both fraternal and identical twins (in this case, they focused on African-American teen girls, a population underrepresented in prior studies), researchers can set up mathematical models designed to probe the influence of environment and genetics. Focusing on alcohol, cigarettes, and marijuana, they found that early use either of alcohol or cigarettes was associated with an increased likelihood of being an early marijuana user, and early drinking was also associated with early cigarette use.  In general, those who had never used a given substance were the least likely to report having used the others -- so in a sense, almost any drug teens try seems to be a gateway drug.

Even more interesting, though, is the more detailed number crunching designed to tease out genetic vs. environmental influences. Inherited influences explained 44% of the variance in initiation of alcohol, 62% for cigarettes, and 77% of the variation in marijuana initiation. Because the confidence intervals (statistical-speak for margin of error) were fairly wide, those numbers should not be taken as gospel, but clearly genes play a major role in susceptibility to early substance use.

There is little doubt that some kids are innately more susceptible to early drug use than others. And a teen who tries one drug -- whatever it is, legal or illegal -- is more likely than his or her peers to try others. These are real issues that parents and educators need to face, and simplistically blaming marijuana as "the gateway drug" won't help them.