A large-scale German study has revealed a clear link between smoking and depression, confirming that current and former smokers are more likely to develop the disorder than those who have never smoked.
The study was conducted by a research group led by the Central Institute of Mental Health (CIMH) in Mannheim, as part of the German National NAKO Study, the largest population-based study in Germany.
The researchers analyzed data from 173,890 participants aged 19 to 72, half of whom were women. The data was collected through personal interviews and standardized questionnaires covering depression diagnoses, current symptoms, living conditions, and smoking habits.
Based on this data, participants were divided into three categories: 81,775 people who had never smoked, 58,004 former smokers, and 34,111 current smokers. Age at smoking initiation, average number of cigarettes smoked daily, and the length of time since quitting for former smokers were also documented.
The results showed that depression was more prevalent among current and former smokers compared to non-smokers, particularly in the middle age group between 40 and 59 years, suggesting a possible role for temporal factors alongside social factors in influencing mental health.
The study also found a clear dose-response relationship; a higher number of cigarettes smoked daily was associated with an increase in the severity of depressive symptoms, at a rate of 0.05 additional symptoms per cigarette. Conversely, starting smoking later in life was associated with a delay in the onset of the first depressive episode.
The results also showed that quitting smoking has tangible positive effects, with longer periods of abstinence being associated with an increase in the time since the last episode of depression.
The researchers pointed out that smoking is the leading preventable cause of premature death globally, contributing to more than 8 million deaths annually, according to the World Health Organization, stressing the importance of preventing the start of smoking and encouraging smokers to quit to improve mental health.
The research team pointed out that the cross-sectional design of the study and the reliance on retrospective data do not allow for definitive causal conclusions, stressing the need for future longitudinal studies that include genetic and imaging data to better understand the mechanisms and potential causal relationships.
The study was published in the journal BMCPublic Health.
