Many studies have demonstrated a positive association between tobacco use and depressive disorders (e.g. Breslau et al. 1991; Klungsoyr et al. 2006; Sihvola et al. 2008). Others have examined the relationship between nicotine dependence (ND) and major depression (MD) or depressive symptoms, with results suggesting that the risk of MD is higher with increasing levels of ND (Breslau & Johnson, 2000; Kessler et al. 2007; Manley et al. 2009). The causes underlying these associations remain unclear. Tobacco use could cause depressive symptoms, and/or depression could lead to tobacco use. Alternatively, the association could be non-causal, i.e. tobacco use and depression could share a common liability, in turn the result of genetic and/or environmental influences. These possibilities are not mutually exclusive.
