Dec. 18, 2009 -- Marijuana is the most widely used illegal drug in the U.S., but little is known about the genes that influence its use. Now new research suggests that many of the same genetic factors that lead people to drink alcohol also lead them to smoke marijuana. Researchers interviewed 2,761 sets of identical and fraternal twins in their mid-20s to mid-30s about their use of alcohol and marijuana. They found little overlap between the environmental factors that influences alcohol and marijuana use, but significant overlap in the genetic factors driving use of the two substances. "We wanted to know if the same genes that affect risk for alcohol dependence also affect risk for marijuana dependence," study researcher Carolyn E. Sartor, PhD, tells WebMD. "Twin studies like this one help us understand how much of a given trait is traceable to genetics or environment."

Dependence on Alcohol and Marijuana The National Institute on Drug Abuse estimates that 21 million Americans aged 12 or older, or about one in 10 people, abuse or are addicted to drugs or alcohol. Of the nearly 7 million people who were classified as dependent on drugs other than alcohol, roughly 4 million abuse marijuana. According to one study, between 8% and 12% of marijuana users are considered dependent on the drug, meaning that they use the drug compulsively even though it often interferes with family, school, work, and recreational activities. Twin studies are conducted to better understand how genes influence behaviors by comparing the actions of identical twins, who have identical genes, with those of fraternal twins, who share just some genes. Results from earlier twin studies, suggest that between 50% and 60% of the variance in alcohol dependence can be explained by genetic factors. Studies also suggest that genes play a major role in marijuana dependence and dependence on other drugs.

Common Genes Drive Dependence The newly published study included 2,761 twin pairs and 736 people who were not twins enrolled in an Australian twin registry. All were between the ages of 24 and 36 when interviewed. Based on their responses, researchers concluded that over 60% of the variance in alcohol and marijuana use and marijuana dependence could be linked to genes. The study appears online today and will be published in the March issue of Alcoholism: Clinical & Experimental Research. "We found considerable overlap between the genes that contribute to marijuana dependencies and alcohol use and dependence," Sartor says. Other researchers are working to identify specific genes that influence substance use and abuse. "What we know from studies like this one is there are many genes working together," substance abuse researcher Christian Hopfer, MD, of the University of Colorado School of Medicine tells WebMD. He says the findings add support to the idea that common genes play a role in all substance abuse.