http://www.sciencedaily.com/releases/2009/12/091219073005.htm?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+sciencedaily+(ScienceDaily%3A+Latest+Science+News)&utm_content=Google+International

'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. A new study has found that use and misuse of alcohol and marijuana are influenced by a common set of genes.

Results will be published in the March 2010 issue of Alcoholism: Clinical & Experimental Research and are currently available at Early View.

"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."

"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 quality of life, interfering with functioning at school or work or leading to problems with family and friends," said Sartor. "Although at least three of six symptoms listed in the Diagnostic and Statistical Manual of Mental Disorders -- Fourth Edition (DSM-IV) are needed to meet full criteria for cannabis (marijuana) dependence … the presence of even one or two of these symptoms could create distress or interfere with day-to-day functioning. 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."' [...]