Major depression and stages of smoking: A longitudinal investigation

Background: Epidemiologic studies have reported an association between major depression and smoking. This prospective study examines the role of depression in smoking progression and cessation, and the role of smoking in first-onset major depression. Methods: Data are from a 5-year longitudinal epidemiologic study of 1007 young adults. Incidence and odds ratios (ORs) are based on the prospective data. Hazards ratios are based on the combined lifetime data and estimated in Cox proportional hazards models with time- dependent covariates. Results: Based on the prospective data, history of major depression at baseline increased significantly the risk for progression to daily smoking (OR, 3.0; 95% confidence interval, 1.1-8.2), but did not decrease significantly smokers' rate of quitting (OR, 0.8; 95% confidence interval, 0.4-1.6). History of daily smoking at baseline increased significantly the risk for major depression (OR, 1.9; 95% confidence interval, 1.1-3.4). These estimates were reduced somewhat when history of early (ie, before age 15 years) conduct problems was controlled. Estimates based on lifetime data were consistent with these results. Conclusions: The observed influences from major depression to subsequent daily smoking and smoking to major depression support the plausibility of shared etiologies. Separate causal mechanisms in each direction might also operate, including self-medication of depressed mood as a factor in smoking progression and neuropharmacologic effects of nicotine and other smoke substances on neurotransmitter systems linked to depression.