Does smoking cause depression?

New and bold claims about how cigarette smoking may cause people to become depressed are made today.

It has long been known that smokers have higher rates of depression than non-smokers.

Depression might lead people to take up smoking – but the new analysis claims tobacco aggravates depression.

Researchers from the University of Otago in New Zealand investigated the link further. They set out to identify causal relationships between smoking and depression.

The team took figures from over 1,000 men and women aged 18, 21 and 25 years.

Smokers had more than twice the rate of depression. Using a computer modelling approach, their analysis supported a pathway in which nicotine addiction leads to increased risk of depression.

In the British Journal of Psychiatry, they suggest two possible routes – one involving common or correlated risk factors, and the second a direct causal link.

They write: "This evidence is consistent with the conclusion that there is a cause and effect relationship between smoking and depression in which cigarette smoking increases the risk of symptoms of depression."

Lead researcher, Professor David Fergusson, said: "The reasons for this relationship are not clear. However, it’s possible that nicotine causes changes to neurotransmitter activity in the brain, leading to an increased risk of depression."

But he adds that the study "should be viewed as suggestive rather than definitive".

Writing in the same journal, Dr Marcus Munafo of Bristol University, UK, reports that cigarette smokers often talk about the antidepressant benefits of smoking. "But evidence suggests that cigarette smoking may itself increase negative affect [emotion], so the causal direction of this association remains unclear," he writes.

"Although increasingly sophisticated analyses of epidemiological data may help to answer this question, observational data can never unequivocally provide evidence of causation," he believes.

Boden, J.M., Fergusson, D. M. and Horwood, L. J. Cigarette smoking and depression: tests of causal linkages using a longitudinal birth cohort. The British Journal of Psychiatry, Vol. 196, June 2010, pp. 440-46.

Munafo, M. R. and Araya, R. Editorial: Cigarette smoking and depression: a question of causation. The British Journal of Psychiatry, Vol. 196, June 2010, pp. 425-26.