LOS ANGELES — Cannabis use in adolescence is associated with the subsequent development of depression—not the reverse, a large longitudinal study shows.
Dr. Hon Ho and his associates from the department of psychiatry at the University of Colorado, Denver, presented the study at the annual meeting of the American Academy of Addiction Psychiatry. He and his colleagues examined data from the National Longitudinal Study of Adolescent Health, a probability sample of people surveyed several times over 14 years on social, economic, psychological, and medical topics.
The sample consisted of 10,778 female and 10,519 male participants who were between the ages of 11 and 21 in the mid-1990s, when the first and second waves of interviews were conducted. About half of the sample was white; 23% were African American; 13%, Hispanic; 8%, Asian; and 2%, Native American.
The median age at Wave 1 was 16, and at Wave 3 (during 2001–2002), 22 years.
Dr. Ho combined responses from Waves 1 and 2, which represented only 1994–1996, and then tracked temporal patterns between first stated use of cannabis and the first indicator of depression based on the Center for Epidemiologic Studies–Depression scale.
Prior cannabis use proved to be a statistically significant predictor of later depression after adjustment for age, sex, race, socioeconomic status, and drug and alcohol use. The relative risk for depression after any cannabis use was 1.27 (95% confidence interval 1.07–1.49), compared with nonusers. That risk increased to 1.33 (1.07–1.64) in people who reported using cannabis on 10 or more occasions. Low socioeconomic status and black, Native American, or Asian race also were predictive of later depression, reported Dr. Ho in an oral presentation of his paper.
The investigators analyzed the reverse scenario: depression at Waves 1 or 2 followed by cannabis use at Wave 3, but found no significant temporal relationship. “Depression did not seem to increase risk of cannabis use at a later time,” he said. The large sample size was a strength of his study, but reliance on self-reported behaviors and the lack of more precise dose information compromised the study's ability to determine causality, rather than a mere association, Dr. Ho said. Nonetheless, the findings do have some policy implications, particularly as local governments consider easing restrictions on marijuana purchasing and use for medicinal purposes, he said.
The apparent association between early cannabis use and later depression is “definitely something we want to consider,” in counseling of adolescents and families, he said.
Dr. Robert L. DuPont, the first director of the National Institute on Drug Abuse, noted that the results “undercut the old view that cannabis [and other drug use] is often a reaction to depression and other psychiatric disorders” and that “cannabis use by adolescents is benign.”
Also in an interview, Dr. Mark S. Gold said the findings are an example of the continual shift in thinking about cannabis. “Research has a way of answering questions, and in this case, and also in the case of mood disorders, substance-induced changes are likely the cause [of dysfunction],” said Dr. Gold, the Donald R. Dizney Eminent Scholar and Distinguished Professor at the University of Florida, Gainesville. “Our work with other drugs of abuse suggests that long-term changes in neuronal and brain function result from chronic or high-dose use and could also occur in those particularly vulnerable by environmental or genetic risks.”
Dr. Ho received U.S. federal funding for his study and had no financial conflicts to report.
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