Alcohol and Marijuana Addiction May Share Genetic Basis

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A new study, to be published in the March 2010 issue of Alcoholism: Clinical & Experimental Research, suggests that genes known to make people susceptible to alcoholism also make them prone to marijuana addiction.

The Study

The study was based on information collected from interviews of almost 6,300 people, including almost 2,800 sets of twins. The results showed that genetics contribute significantly to both alcoholism and marijuana dependence. According to the study, about 60% of the likelihood of becoming a heavy drinker or dependent on marijuana is related to genetics, while about 50% of the likelihood of becoming an alcoholic can be attributed to genes. Although specific genes likely influence addiction to different substances, it appears there is also a common set of genes that influences both marijuana and alcohol addiction.

Despite indications of an obvious genetic predisposition toward substance abuse, study author Carolyn Sartor notes that no one is predestined to abuse drugs. Even with the genetic influence, 40-60% of the likelihood of dependence to alcohol or marijuana may be due to external, environmental factors.

What Does This Mean?

Past research of addictive drugs has generally focused on the properties of just one drug, considering substances like marijuana, alcohol, tobacco, and cocaine separately. A study such as this one, in which more than one substance is examined, suggests that similar genes play a role in many types of substance abuse. As Dr. Christian Hopfer, an associate professor at the University of Colorado School of Medicine, points out, "There is a lot of evidence that if you have trouble with one substance you will have trouble with others. Twin data shows that the genetic effects may be across substances."

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