Persistent smokers appear to be at increased risk for becoming depressed compared to never smokers, results of a long-term study of Finnish twins suggest.

On the other hand, this association was not seen in individuals who stopped smoking many years ago.

"Although nicotine in cigarettes has some mood-elevating properties, in the long-run chronic exposure to cigarette smoke may have a more important role in the etiology of depressive symptoms," lead author Dr Tellervo Korhonen from the University of Helsinki told Reuters Health.

The results are based on 4,000 male and 5,000 female Finnish twins, whose health and health behavior were monitored for 15 years.

The results suggest that persistent chronic smoking predicts the development of depressive symptoms.

However, when adjusted for other factors associated with depression, the elevated risk of depression with persistent smoking remained significant only among men.

There was also evidence that smokers who had quit were also at increased risk of depression, but only in the short term.

Smokers who quit and remained off cigarettes in the long run did not have an increased risk for depression compared with never smokers.

"This may reflect a relatively long recovery process from the adverse effects of cigarette smoking, Korhonen said in a statement.

"When people start smoking, the immediate effects of nicotine in the brain are rewarding and pleasurable," Korhonen explained.

"This suggests self-medication, where a person who has mood problems seeks relief via cigarette."

Because addiction to nicotine is as strong as an addiction to heroin, abstinence is difficult.

"Smokers who are vulnerable to depression may need specific pharmacological treatment and behavioral support to overcome the earlier phase of abstinence," Korhonen said. After that, "their chances to quit successfully improve."

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