Twin Study Links Marijuana Abuse, Suicide,And Depression

Men and women who smoked marijuana before age 17 are 3.5 times as likely to attempt suicide as those who started later. Individuals who are dependent on marijuana have a higher risk than nondependent individuals of experiencing major depressive disorder and suicidal thoughts and behaviors. The researchers who discovered these relationships, in a recent NIDA-funded large-scale epidemiological study, say that although the causes are not clear, their findings demonstrate the importance of considering associated mental health issues in the treatment and prevention of marijuana abuse.
Dr. Michael Lynskey and colleagues at the Washington University School of Medicine in St. Louis, Missouri, gathered data from four groups of same-sex twin pairs (508 identical, 493 fraternal; 518 female, 483 male) enrolled in the Australian Twin Registry. The groups and findings were:
Among the 277 pairs who were discordant for marijuana dependence (that is, one twin but not the other met the criteria for a diagnosis of marijuana dependence), the dependent twins were 2.9 times as likely as their nondependent co-twins to think about suicide without attempting it, and 2.5 times as likely to make a suicide attempt;
Among the 311 pairs discordant for early marijuana initiation (just one twin in each pair smoked marijuana before age 17), the early initiators were 3.5 times as likely as their twins to attempt suicide, but no more likely to suffer a major depressive disorder (MDD);
Among the 156 pairs discordant for diagnosis of MDD before age 17, fraternal but not identical twins with early diagnosis of MDD were 9.5 times as likely to develop marijuana dependence; and
Among the 257 pairs discordant for having suicidal thoughts before age 17, fraternal but not identical twins with early suicidal thoughts were 5.5 times as likely as their twins to become dependent on marijuana.
"Overall, the associations between marijuana abuse and depressive disorders suggest a relationship that is contributory but not necessarily causal. Depressive disorders in and of themselves do not cause people to abuse marijuana, and marijuana abuse and dependence do not of themselves cause depression or suicidal behavior," Dr. Lynskey says. "Nevertheless, clinicians treating patients for one disorder should take the other into account at initial assessment and throughout treatment. In the context of treatment, both need to be addressed, because it is not necessarily the case that eliminating one disorder will get rid of the other." The fact that two of the relationships were observed in fraternal but not identical twins suggests that the experiences related in each—marijuana dependence and MDD, and marijuana dependence and suicidal thoughts—may share a common underlying genetic basis, notes Dr. Lynskey.
The associations identified in this study are complex, but point to a simple policy implication, observes Dr. Lynskey. "It is important to see that prevention efforts aimed at one disorder may well have the additional benefit of preventing or reducing the other," he says.
"Drug abuse and depression co-occur at rates much greater than chance and constitute a serious public health concern," says Dr. Naimah Weinberg of NIDA's Division of Epidemiology, Services and Prevention Research. "Understanding how each disorder may contribute to the development and course of the other, and what factors may underlie their co-occurrence, has important implications for prevention and treatment of these disabling conditions. Genetic epidemiologic approaches, such as those applied by Dr. Lynskey and his colleagues, are very powerful tools to help parse out the etiologic relationships between co-occurring disorders."