The study, carried out by researchers from the University of Otago in New Zealand, followed over 1,000 people. At the ages of 18, 21 and 25, the participants were asked about their smoking habits and whether they had symptoms of depression.
The researchers found a strong association between smoking and depression. People who were dependent on nicotine were more than twice as likely to have symptoms of depression as those who were not nicotine dependent.
The researchers looked at this relationship in more detail using a sophisticated statistical technique called structural equation modelling (SEM). This analysis showed that smoking increases the risk of developing depressive symptoms, rather than people being more likely to smoke because they’re depressed.
Commenting on the results, lead researcher Professor David Fergusson said: “Our findings are consistent with the conclusion that there is a cause and effect relationship between smoking and depression, in which cigarette smoking increases the risk of developing symptoms of depression.
“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.”
Professor Fergusson and colleagues do emphasise that their study does not prove that smoking causes depression, and said that the study “should be viewed as suggestive rather than definitive”.
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Boden JM, Fergusson DM and Horwood LJ (2010) Cigarette smoking and depression: tests of causal linkages using a longitudinal birth cohort, British Journal of Psychiatry, 196:440-446