“It appears that about one-third of genetic influences on alcohol and marijuana use and dependence can be traced to the same set of genes,” says first author Carolyn E. Sartor, Ph.D. “Other genetic contributions are specific to alcohol or marijuana.”
The study results will be published in the March 2010 issue of the journal Alcoholism: Clinical & Experimental Research, and they are available online through the journal’s Early View.
Sartor, a research instructor in the Department of Psychiatry, says in both alcohol and marijuana, the majority of genetic factors that contribute to use also contribute to the development of dependence symptoms. She and her colleagues studied more than 6,200 Australian twins between the ages of 24 and 36. The twins were asked about the amount of alcohol and marijuana they had consumed over their lifetimes, their frequency of use, when they used the most, and questions that helped scientists determine whether the twins had symptoms of alcohol or marijuana dependence.
By studying twins, it’s possible to get a handle on the roles played by genetic and environmental factors involved in behaviors such as drinking alcohol or using marijuana. Identical twins share 100 percent of their genetic material, and fraternal twins share about half of their segregating genes. So when identical twin pairs are found to be more similar in a given behavior than fraternal twin pairs, it’s likely genes are playing a significant role. The researchers used the twin model to estimate the contributions of genes and environment in alcohol and marijuana use and dependence.
They classified alcohol users according to how much they drank. Those who were heavier users of alcohol were more likely to use marijuana, too. The survey found that almost everyone drank alcohol, at least occasionally.
Marijuana is the most commonly used illicit drug in the United States. Roughly 8 percent to 12 percent of marijuana users are considered dependent and, as with alcohol, the severity of their symptoms increases with heavier use.
“In a 2008 survey of high-school students in the United States, 41.8 percent of 12th graders reported having used marijuana,” Sartor says. “Most had used the drug on just a few occasions, but more than 5 percent reported using it daily in the month preceding the survey.”
Heavy marijuana or alcohol use is closely linked to dependence. Not long ago, many believed marijuana was not an addictive substance like alcohol or nicotine, but Sartor says that’s incorrect.
“There is a myth that cannabis is harmless, and although there is plenty of recreational use of the drug that does not lead to problems, like any other drug, you can misuse it to the point that it interferes with day-to-day functioning,” she explains. “For about 10 percent of the people who ever try marijuana, that’s what happens.”
Although studies involving genetic contributions to alcohol dependence have been ongoing for close to 30 years, similar studies involving marijuana have emerged much more recently. Sartor believes ultimately environmental factors linked to alcohol and marijuana use may be key to effective therapies for abuse and dependence.
“Interestingly, the environmental influences on use and dependence did not appear to be the same for alcohol as for marijuana,” Sartor says. “The overlap in genetic influences on marijuana-dependence symptoms and alcohol-dependence symptoms tells us that individuals with family histories of alcohol-related problems are at increased risk for developing problem marijuana use and that a family history of marijuana-related problems is an indicator of elevated risk for alcohol misuse. But when we talk about prevention and treatment — since it’s the environmental components rather than the genetics that we can manipulate — we may need to focus on those substance-specific environmental influences to help people avoid becoming dependent on alcohol or marijuana.”
Sartor CE, Grant JD, Bucholz KK, Madden PAF, Heath AC, Agrawal A, Whitfield JB, Statham DJ, Martin NG, Lynskey MT. Common genetic contributions to alcohol and cannabis use and dependence symptomatology. Alcoholism: Clinical & Experimental Research, vol. 34 (3), March 2010
This study was supported by grants from the National Institute on Alcohol Abuse and Alcoholism and the National Institute on Drug Abuse of the National Institutes of Health.
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