The likelihood of suffering major depression seems to be increased among smokers, especially those who smoke heavily, study findings suggest.

Researchers in Norway who followed a population-based group of adults for 11 years found that those who smoked were more likely than non-smokers to become depressed, and the risk climbed in tandem with the number of cigarettes smokers puffed each day.

Heavy smokers - those who burned through more than 20 cigarettes a day - were four times more likely than people who’d never smoked to develop depression.

A number of factors the researchers considered - including physical health, exercise and stressful life events - failed to explain the link between smoking and later depression. This suggests, they say, that smoking may directly contribute to the development of the mood disorder.

For instance, nicotine may over time change brain levels of the emotion-related chemical serotonin, which appears to be reduced in people with depression, the study’s lead author, Dr. Ole Klungsoyr, told Reuters Health.

Klungsoyr and colleagues at the University of Oslo report their findings in the American Journal of Epidemiology.

A number of studies have found that smokers have higher-than-average rates of depression, but an unanswered question has been which comes first. People who are under chronic stress or who are prone to depressive symptoms, for example, may be particularly likely to take up smoking.

The current study, however, found no evidence that past depression symptoms were strongly linked to subsequent smoking. Only 15 percent of depression cases arose before study participants started smoking, the researchers report.

Their findings are based on interviews, conducted 11 years apart, with 1,190 men and women age 18 and older. All participants answered questions about their lifestyle and mental health, and were assessed for clinical depression at both time points.

Overall, the risk of developing depression by the second interview climbed along with the number of cigarettes a smoker had each day. Potential explanations like physical health problems or greater stress among smokers did not change the relationship between smoking and depression.

It’s possible, Klungsoyr said, that other factors the study could not consider - like personality traits that make a person prone to both depression and smoking - are responsible. More studies are needed to replicate the current findings, he added.

But in the meantime, Klungsoyr said, the possible connection to depression offers yet another reason to quit or never take up smoking. “It is one more argument on an already very long list,” he noted.

SOURCE: American Journal of Epidemiology, March 1, 2006.