Genetic Risk Factors for both Marijuana and Alcohol Abuse Similar

A new study has found that use and misuse of alcohol and marijuana are influenced by a common set of genes. The results are in the March 2010 issue of Alcoholism: Clinical & Experimental Research.

Marijuana is the most commonly used illicit drug in the United States. Roughly eight to 12 percent of marijuana users are considered “dependent” and, just like alcohol, the severity of symptoms increases with heavier use.

Results from a large annual survey of high-school students show that in 2008, 41.8 percent of 12th graders reported having used marijuana, explained 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,” she said.

“The active ingredient in marijuana is THC, which mimics natural cannabinoids that the brain produces,” added Christian Hopfer, associate professor at the University of Colorado School of Medicine. “The cannabinoid 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.”

Like any drug, marijuana can be used in a way that negatively impacts a student’s quality-of-life, interfering with functioning at school or work or leading to problems with family and friends, researchers found. There is strong evidence for a genetic component to use and dependence on marijuana as well as alcohol, and the use (and misuse) of these substances frequently occur together.

Researchers examined 6,257 individuals (2,761 complete twin pairs and 735 singletons) listed in the Australian Twin Registry, 24 to 36 years of age. Alcohol and marijuana use histories were gathered in telephone diagnostic interviews and used to derive levels of alcohol consumption, frequency of marijuana use, and alcohol and cannabis dependence symptoms listed in the Diagnostic and Statistical Manual of Mental Disorders, fourth edition. (Better known as the DSM-IV, the manual is published by the American Psychiatric Association and covers all mental health disorders for both children and adults. It also lists known causes of these disorders, statistics in terms of gender, age at onset, and prognosis as well as some research concerning the optimal treatment approaches.

The researchers’ findings indicate many of the same genetic factors that contribute to alcohol use also contribute to marijuana use. Likewise, alcohol dependence symptoms and cannabis dependence symptoms can be traced to some of the same genetic influences. For both alcohol and marijuana, the majority of genetic factors that contribute to use also contribute to dependence symptoms.

Hopfer concluded: “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.”