Marijuana use and depression among adults: testing for causal associations

Aim: To determine whether marijuana use predicts later development of depression after accounting for differences between users and non-users of marijuana. Design: An ongoing longitudinal survey of 12 686 men and women beginning in 1979. Setting: The National Longitudinal Survey of Youth of 1979, a nationally representative sample from the United States. Participants: A total of 8759 adults (age range 2937 years) interviewed in 1994 had complete data on past-year marijuana use and current depression. Measurements: Self-reported past-year marijuana use was tested as an independent predictor of later adult depression using the Center for Epidemiologic StudiesDepression questionnaire. Individual's propensity to use marijuana was calculated using over 50 baseline covariates. Findings: Before adjusting for group differences, the odds of current depression among past-year marijuana users is 1.4 times higher (95% CI: 1.1, 1.9) than the odds of depression among the non-using comparison group. After adjustment, the odds of current depression among past-year marijuana users is only 1.1 times higher than the comparison group (95% CI: 0.8, 1.7). Similarly, adjustment eliminates significant associations between marijuana use and depression in four additional analyses: heavy marijuana use as the risk factor, stratifying by either gender or age, and using a 4-year lag-time between marijuana use and depression. Conclusions: After adjusting for differences in baseline risk factors of marijuana use and depression, past-year marijuana use does not significantly predict later development of depression. These findings are discussed in terms of their relevance for understanding possible causal effects of marijuana use on depression. (Author' s abstract)