Abuse of Alcohol, Pot Linked to Same Genes
Study of Twins Shows Similar Genetic Factors Lead to Drinking and Marijuana Use
WebMD News Archive
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
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
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.