New Findings Support Theory that Smoking Causes Depression
Medically reviewed by Paul Sietes, MD.
Current heavy smokers are at three times greater risk for major depression compared to former heavy smokers, according to a study published in the Journal of Psychiatric Research.
Although the link between smoking and depression is well-documented, the results support the causal hypothesis regarding smoking and depression rather than simply the “shared-vulnerability” hypothesis.
“Under the shared-vulnerability hypothesis, ever-heavy smokers may be expected to have similar elevated risk for major depressive episode irrespective of their smoking status during followup,” said Salma Khaled, Ph.D. “Our results point to the contrary.”
Khaled, who was employed by the Mental Health Center for Research and Teaching while the research was conducted, and a team evaluated information from 3,824 adults in the Canadian National Population Health Survey.
Participants in the survey were interviewed between 1994 and 1995 and were followed with new interviews conducted every second year through 2006-2007.
To be included in the study, participants had to have maintained their smoking status as current, former, or never smokers throughout the entire survey followup. “Heavy” smokers were identified as those who smoked 20 or more cigarettes per day.
“Ever-heavy smokers (current and former) may share similar genetic, behavioral, and environmental vulnerabilities, at least for heavy smoking initiation,” said Khaled, who is now at the University of Calgary.
If these factors were wholly to blame for depression — as given by the shared-vulnerability hypothesis — then we would see former-heavy smokers and current smokers with an equal likelihood of having a major depressive episode (MDE), Khaled reasoned.
“However, if the persistence of the exposure (current as opposed to former) had the dominant effect on the risk for MDE, then current-heavy smokers would be expected to have higher risks of MDE relative to former-heavy smokers,” said Khaled.
The authors found that, overall, the 12-year risk of MDE for the entire sample was 13.2 percent.
When split by smoking status, the risk of MDE among current-heavy smokers was 26.7 percent; among former-heavy smokers it was 7.1 percent, and among those who never smoked it was 12.2 percent.
These statistics showed a significant hazard ratio of 3.1 for current heavy smokers, compared with former smokers, even after adjusting for age, sex, and stress.
Furthermore, the hazard ratios for MDE among former-heavy smokers, compared with current smokers, steadily decreased over time since quitting. The hazard ratio dropped from 0.5 for those who quit between 1 and 5 years ago to 0.2 among those who quit smoking more than 21 years ago.
“Our findings are consistent with the view that the heavy smoking-to-major depression pathway is causal in nature, rather than mainly due to confounding by shared vulnerability factors,” noted Khaled and her colleagues.
Nevertheless, she said that “shared vulnerability factors including genetic vulnerability in the context of smoking and depression may not be limited to smoking initiation and heavy smoking onset, but may also influence the ability to quit smoking and maintain smoking cessation.”
Source: Journal of Psychiatric Research