Patients with major depressive disorder (MDD) often have comorbid psychiatric conditions that affect their smoking rates and may make it difficult to tease out how much smoking is due to MDD, as opposed to these other conditions. This finding came from the Vantaa Depression Study, a collaborative depression research project between the Mood, Depression, and Suicidal Behavior Unit of the National Institute for Health and Welfare in Helsinki, Finland, and the Department of Psychiatry of Helsinki University Central Hospital at Peijas Hospital, which provides secondary-care psychiatric services to all of the approximately 169,000 residents of the municipality of Vantaa. After baseline evaluation, participants were evaluated again at six months, 18 months, and five years.
The researchers concluded that smoking among MDD patients is associated with several important clinical characteristics, particularly substance-use disorders, and with personality factors (personality disorders and neuroticism), which may markedly confound research on the impact of smoking. Rather than depression and smoking covarying or predicting each other, smoking and alcohol-use disorders each have strong autoregressive tendencies. These findings are more consistent with common factors causing their association than with depression resulting in increased smoking or smoking inducing onset of depression. ■