Tobacco smoking as a risk factor for depression. A 26-year population-based follow-up study

BackgroundA key question regarding the documented association between smoking and depression is whether it reflects a causal influence of smoking on depression; however, only a limited number of longitudinal studies exist in the literature, all of which have relatively short time frames. The purpose was to prospectively assess the risk of depression according to daily tobacco consumption in a Danish longitudinal study.MethodsA prospective cohort study, the Copenhagen City Heart Study (n = 18,146), was analyzed with up to 26 years of follow-up. It contains three updated data sets on tobacco consumption and potential confounding factors. The study population was linked to Danish hospital registers to detect registrations with depression. Individuals with depression before baseline were excluded.ResultsWomen smoking more than 10 g of tobacco per day were at significantly increased risk of depression compared to women who did not smoke. The adjusted risk of depression among women smoking 11–20 g per day was 1.74 (CI:1.33–2.27) and 2.17 (CI:1.45–3.26) among women smoking more than 20 g per day. For men, there was an increased risk of depression for those smoking more than 20 g per day (HR = 1.90; CI:1.05–3.44). All tests for linear trend were significant (all p < 0.01). The estimates remained significant in sensitivity analyses aiming to eliminate reverse causation, and in analyses based on a reduced sample without individuals with chronic diseases or psychiatric disorders other than depression.ConclusionThe study suggests that smoking is associated with increased risk of developing depression. This underlines the potentially harmful consequences of smoking for mental health and supports efforts to prevent and stop smoking.