In a recent study by Khaled et al (2012), the authors explored the association between smoking and major depression. They examined 12-year risk of major depression among persistent heavy smokers, as compared to former heavy smokers, using data from the National Population Health Survey (NPHS) in Canada. The authors found that current heavy smokers were over three times more likely than former heavy smokers to experience an episode of major depression, even after accounting for differences in age, sex, and stress. Further, former heavy smokers became increasingly less likely to suffer from major depression the longer they were abstinent from smoking, pointing to the benefits of smoking cessation maintenance.
Khaled SM, Bulloch AG, Williams JV, Hill JC, Lavorato DH, Patten SB. Persistent heavy smoking as risk factor for major depression (MD) incidence – evidence from a longitudinal Canadian cohort of the National Population Health Survey. J Psychiatr Res 2012, 46(4):436-443 (abstract)