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 DSM-IV alcohol and cannabis dependence symptoms.
"Our findings indicate that … many of the same genetic factors that contribute to alcohol use also contribute to marijuana use," said Carolyn E. Sartor, corresponding author for the study. "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."
"In other words, the genetic influences on drug use are not specific to individual drugs, but seem to influence a general tendency to engage in drug use. This is important to note because there is a tendency to study drugs in isolation – alcohol, tobacco, marijuana, cocaine, etc. These findings add support to the notion of common mechanisms underlying all addictions," added University of Colorado researcher Christian Hopfer.
"The fact that very little of the environmental influences on alcohol and marijuana use, or on alcohol and cannabis dependence symptoms, could be traced to common sources indicates that there may be important distinctions between those environmental factors that influence alcohol-related outcomes and those that influence marijuana-related outcomes," said Sartor. "Identifying alcohol-and marijuana-specific risk factors is an important next step in this line of research."
"Marijuana research is relatively sparse compared to alcohol or nicotine research," added Hopfer. "However, if you look at reports of at least adolescents and young people using, it becomes clear that marijuana use, including daily marijuana use, is quite common and the effects of this are not well understood. 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."