Discussion of the paper 'Tobacco smoking as a risk factor for major depressive disorder: population-based study' by J.A. Pasco, L.J. Williams, F.N.Jacka, F. Ng, M.J. Henry, G.C.Nicholson, M.A.Kotowicz, M.Berk. British Journal of Psychiatry (2008) 193, 322-326.

Smoking is an important public health problem against which many prevention strategies have been undertaken. However, smoking is disproportionately prevalent in people with diagnosed mental illness. It is often considered as a secondary therapeutic target in the treatment of mental disorders. It is becoming increasingly clear that smoking is not harmless to mental health, and may in fact potentiate mental illness and contribute to its onset. At a neurobiological level, this may be related to nicotine's effect on the regulation of monoamine neurotransmission, including dopamine, through the proliferation of cholinergic pathways. Smoking may, according to researchers of this phenomenon, increase the likelihood of mood disorders.

Researchers already have evidence that smoking increases the risk of depression. Studies show that smoking precedes the onset of depression. However, we have limited data from longitudinal studies on the above consequences of nicotine dependence. In the epidemiological study described by the authors, we investigated the status of nicotine dependence as a risk factor for depression using data from longitudinal studies over the past 10 years and data from cross-sectional studies.

Subjects for the present study were drawn from a research programme initially identifying the epidemiology of osteoporosis in Australian women, which was subsequently expanded to include psychiatric disorders and somatic illness. The study began in 1994-1997 and continued for 10 years. Data were collected on lifestyle, smoking, alcohol consumption, levels of usual physical activity and exposure to disease. Smoking was defined here as regularly smoking more than one or two cigarettes per day for the past six months. Ordinary physical activity was defined as positive when the respondent indicated that they walk, move or work vigorously or participate in exercise. Exposure was defined as cardiovascular disease and also first- and second-type diabetes.

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A clinical interview was also conducted to detect women who had depression and to determine the age of onset. Nicotine smoking was recognised if it occurred before the diagnosis of depression. Among the women interviewed ( N=1043), 237 were diagnosed with depression, while 806 were not diagnosed with depression.