title: Persistent heavy smoking as risk factor for major depression (MD) incidence--evidence from a longitudinal Canadian cohort of the National Population Health Survey.
abstract: BACKGROUND
  Reports of bidirectional associations between smoking and major depression (MD) have been interpreted as providing evidence for confounding by shared-vulnerability factors (SV) that predispose individuals to both conditions. If this is true, then smoking cessation may not reduce the risk of MD. From clinical practice and public health perspectives, the long-term outcomes associated with smoking persistence and cessation are potentially important and deserve exploration. To this end, the 12-year risk of MD in persistent heavy smokers and abstainers who were former-heavy smokers with and without adjustment for potential confounders were compared.
  
  
  METHODS
  Follow-up data from the National Population Health Survey (NPHS) was used. Multinomial logistic (ML) models were fit to identify potential confounders. Using proportional hazard (PH) models, unadjusted and adjusted hazard ratios (HRs) for MD outcome were estimated for different smoking patterns.
  
  
  RESULTS
  The unadjusted HR relating the risk of MD among current-heavy versus former-heavy smokers was 4.3 (95% CI: 2.6-6.9, p < 0.001). Current-heavy smoking predicted onset of MD (HR = 3.1, 95% CI: 1.9-5.2, p < 0.001) even after adjustment for age, sex and stress - the main confounders. However, this was not the case for the never, former-light, and current-light categories. Evidence of decreased risk of MD among former-heavy relative to current-heavy smokers as function of smoking cessation maintenance time was also found.
  
  
  CONCLUSIONS
  Contrary to common beliefs about the benefits of smoking for mental health, our results suggest that current-heavy rather than ever-heavy smoking is a major determinant of MD risk and point towards the benefits of smoking cessation maintenance.
authors.affiliation: Department of Community Health Sciences, Mental Health Center for Research and Teaching, Canada. smkhalid@ucalgary.ca
authors.forename: Salma M
authors.initials: SM
authors.lastname: Khaled
authors.forename: Andrew G
authors.initials: AG
authors.lastname: Bulloch
authors.forename: Jeanne V A
authors.initials: JV
authors.lastname: Williams
authors.forename: Jeremy C
authors.initials: JC
authors.lastname: Hill
authors.forename: Dina H
authors.initials: DH
authors.lastname: Lavorato
authors.forename: Scott B
authors.initials: SB
authors.lastname: Patten
mesh_terms: D000328:Adult; D002170:Canada / Q000453:epidemiology; D015331:Cohort Studies; D018672:Community Health Planning; D003865:Major Depressive Disorder / Q000453:epidemiology*; D005260:Female; D006306:Health Surveys; D006801:Humans; D015994:Incidence; D016015:Logistic Models; D008297:Male; D008875:Middle Aged; D016016:Proportional Hazards Models; D011569:Psychiatric Status Rating Scales; D012189:Retrospective Studies; D012307:Risk Factors; D012907:Smoking / Q000453:epidemiology*; D055815:Young Adult