The researchers found that ex-smokers had an elevated risk of depressive symptoms in the short run, but in the long term the risk declines to the level of those who have never smoked, according to the study published in Psychological Medicine.

Dr. Tellervo Korhonen of the University of Helsinki used a database of 4,000 male and 5,000 female twins whose health and health behavior were tracked for 15 years. Data on smoking behavior and changes in it between 1975 and 1981 were analyzed as a predictor of depressive symptoms measured in 1990.

Persistent chronic cigarette smoking predicts depressive symptoms, but when adjusted for other factors associated with depression, the elevated risk of persistent smoking remained significant among men only, according to Korhonen.

"The mechanisms underlying the association between smoking and depression are very complicated," Korhonen said in a statement. "Although nicotine as such may have short-term positive effects on concentration and possibly on mood, long-term exposure to tobacco smoke may be one risk factor in development of depression."