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.
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聽<聽0.001). Current-heavy smoking predicted onset of MD (HR聽=聽3.1, 95%CI: 1.9-5.2, p聽<聽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.
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.