Does smoking cessation result in improved mental health? A comparison of regression modelling and propensity score matching

Objectives: Smokers report that smoking is therapeutic; a recent meta-analysis suggests the contrary. However, the association in that review may be explained by group-membership bias and confounding. Propensity score matching (PSM) aims to produce causal estimates from observational data. We examined the association between cessation and change in mental health before and after PSM. Design: A secondary analysis of prospective data from 5 placebo-controlled randomised trials for smoking reduction. Participants: All participants were adult smokers and had smoked for at least 3 years. Participants were excluded if they were pregnant, breast feeding, under psychiatric care, deemed to be unfit by a general practitioner or part of a cessation programme. In total, 937 participants provided smoking data at both 6-month and 12-month follow-ups. Of these, 68 were confirmed as abstinent at both 6 and 12 months and 589 as continuous smokers at both follow-ups. Primary outcome: Change in mental health (36-item Short Form Survey (SF-36), scored 0-100) from baseline (while all participants were smokers) to 12-month follow-up (after cessation) was compared between quitters and continuing smokers with and without adjustment, and after PSM. Results: Before matching, quitters' mental health scores improved compared with continuing smokers', the mean difference and 95% CI was 5.5 (1.6 to 9.4). After adjustment, the difference was 4.5 (0.6 to 8.5), and after PSM, the difference was 3.4 (-2.2 to 8.9). Conclusions: Improvements in mental health after smoking cessation may be partly but not completely explained by group membership bias and confounding.