A longitudinal cohort study from New Zealand has suggested a unidirectional cause and effect relationship between cigarette smoking and depression, in which smoking increases the risk of developing symptoms of depression.

The Christchurch Health and Development Study has followed up a group of 1,265 children who were born in 1977. Participants were studied at ages 18, 21 and 25, when 79-81% of the original cohort attended.

At each follow-up, a standardised diagnostic interview (CIDI) was used to count the number of DSM-IV depression symptoms experienced during the preceding 12 months. Participants who said that they had smoked within the past month were also questioned about the DSM-IV symptoms of nicotine dependence.

Smokers with at least five symptoms of nicotine dependence reported more than twice as many symptoms of depression as non-smokers (incidence rate ratio 2.13). After adjustment for genetic and environmental confounding factors, the magnitude of the association was reduced but remained significant.

This suggests that the association between smoking and depression is in part due to a causal relationship. There are three possibilities: either smoking may increase the risk of depression, or depressed patients may self-medicate with nicotine, or there may be a reciprocal relationship in which both occur.

Structural equation models were constructed for each of these three options, of which the first (a unidirectional model from nicotine dependence to depressive symptoms) best fitted the data.

There are two implications for practice. When case finding among patients with chronic conditions, smoking is yet another important risk factor to add to the list of functional impairment, previous depression, physical inactivity, obesity and maladjustment.

More controversially, it has been suggested that we should routinely target smoking cessation when treating depressed smokers.¹ Between one in three and one in four depressed smokers are willing to attempt cessation,² and a meta-analysis found that they are just as likely to succeed as smokers who are not depressed.³ However, stopping smoking may increase the risk of depression relapse, failed cessation attempts may have a detrimental psychological impact and there is as yet no good evidence that long-term abstinence can achieve a beneficial effect on mood.²

Dr Phillip Bland