Marijuana and alcohol use and abuse share common gene

Kathleen Blanchard's picture
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New findings show that marijuana and alcohol use and abuse share a common gene. As the result of studying twins and singletons, taken from 6257 individuals who were listed in the Australian Twin Registry, researchers have found that dependence on marijuana and alcohol seem to share a common gene.

The significance of the findings comes from studying drugs in isolation. According to Christian Hopfer, associate professor at the University of Colorado School of Medicine, "In other words, the genetic influences on drug use are not specific to individual drugs, but seem to influence a general tendency to engage in drug use. This is important to note because there is a tendency to study drugs in isolation – alcohol, tobacco, marijuana, cocaine, etc. These findings add support to the notion of common mechanisms underlying all addictions."

Marijuana is one of the most common drugs used illicitly in the United States. A 2008 survey of high school students revealed that 41.8 percent of 12th graders reported using marijuana. According to Carolyn E. Sartor, a research instructor at Washington University School of Medicine and corresponding author for the study, "Although many may have used the drug on only a few occasions, 5.4 percent of 12th graders reported using it daily within the preceding month." Hopfer explains that one in 12 marijuana users will develop a dependency.

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Hopfer says, "The active ingredient in marijuana is THC, which mimics natural cannabinoids that the brain produces. The cannabinoid A system is critical for learning, memory, appetite, and pain perception. Most users of marijuana will not develop an 'addiction' to it, but perhaps one in 12 will. What is not commonly appreciated about marijuana use is that strong evidence has emerged that it increases the risk of developing mental illnesses and possibly exacerbates pre-existing mental illnesses."

The study found that many of the same genetic factors that increase susceptibility to alcohol abuse can also increase the chances of dependency on marijuana. "For both alcohol and marijuana, the majority of genetic factors that contribute to use also contribute to dependence symptoms", explains Sartor.

"Marijuana research is relatively sparse compared to alcohol or nicotine research", added Hopfer. "However, if you look at reports of at least adolescents and young people using, it becomes clear that marijuana use, including daily marijuana use, is quite common and the effects of this are not well understood. The mental illness/marijuana connection has not received much press, although I think the evidence has grown substantially that marijuana is a causal risk factor for the development of mental illness."

The researchers say there seems to be little evidence that environmental influences are shared between alcohol and marijuana use. "Identifying alcohol- and marijuana-specific risk factors is an important next step in this line of research." There may be specific environmental influences that determine who is at risk for alcohol abuse, separate from influences that lead to marijuana dependency." For the current study, the researchers gathered information from telephone interviews among 2,761 complete twin pairs and 735 singletons, and alcohol and marijuana dependence symptoms.The findings point to a common gene that influences alcohol and marijuana use and abuse.

DOI: 10.1111/j.1530-0277.2009.01120.x

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