A study funded by the National Institute on Drug Abuse (NIDA), National Institutes of Health, demonstrated that identical male twins were more likely than non-identical male twins to report similar responses to marijuana use, indicating a genetic basis for their sensations. Identical twins share all of their genes, and fraternal twins share about half.
NIDA Director Dr. Alan I. Leshner said, "The finding that genetic factors contribute to how an individual feels after using marijuana opens new avenues for prevention and treatment research. And it further emphasizes that drug use and addiction are not simply social problems, but are health issues affected by an individual's biological state."
Dr. Michael Lyons, Dr. Ming Tsuang, and their colleagues at the Harvard Medical School in Boston, compared identical and fraternal twins on a series of questions in a detailed confidential interview of how "pleasant" or "unpleasant" they felt after smoking marijuana. The identical twins answered similarly while the fraternal twins did not. The results indicate that genes have a significant influence on individual pleasant or unpleasant responses to the effects of marijuana.
Environmental factors such as the availability of marijuana, expectations about how the drug would affect them, the influence of friends and social contacts, and other factors that would be different even for identical twins were also found to have an important effect; however, it was also discovered that the twins' shared or family environment before age 18 had no detectable influence on their response to marijuana.
Taking the environmental and genetic influences together, these results suggest that while exposure to marijuana by factors such as social contacts are important, there are individual differences, perhaps in the brain's reward system, associated with genetic factors that influence whether one will continue using marijuana. Environmental factors may lead to an individual's experimenting with a drug, according to Dr. Tsuang, but heredity appears to hold the key to whether or not an individual will continue to use or abuse the drug.
More than 8,000 male twins, drawn from the Vietnam Era Twin Registry, participated in the study. Of these, 352 pairs of identical twins and 255 pairs of fraternal twins each reported having used marijuana more than 5 times. These individuals were asked if they had experienced each of 23 possible reactions, ranging from confused or paranoid ("negative" responses) to relaxed or mellow ("positive" responses) after marijuana use. Not surprisingly, individuals who perceived marijuana use as pleasurable tended to use it more frequently and those who found the experience unpleasant tended to use it less often. The experienced reactions to marijuana use were more similar for identical twins than fraternal twins.
"This study confirms our hypothesis that genes map an individual's physiology in ways that determine how the individual will feel when using marijuana," said Dr. Lyons. The specific gene or genes involved could not be identified from this study, but it is speculated they are those involved in the brain's reward system.
These physiological differences coupled with the observation that individuals who find pleasure in using marijuana are more likely to use it repeatedly lead to the conclusion that heredity plays a significant role in determining susceptibility to continuing marijuana abuse. The authors postulate that their finding might apply also to use of other substances as well, such as alcohol and cocaine.
NIDA supports more than 85 percent of the world's research on the health aspects of drug abuse and addiction. The Institute also carries out a large variety of programs to ensure the rapid dissemination of research information and its implementation in policy and practice. Further information on NIDA research and other activities can be found on the NIDA Home Page at http://www.nida.nih.gov.