The Effect Of Smokers Transitioning To E-Cigarettes On Physical And Mental Health: An Emulated Trial Using Longitudinal Data.

IntroductionTobacco smoking remains a leading cause of preventable death in the UK. Although e-cigarettes are promoted as a harm-reduction option, longitudinal evidence on short-term health outcomes across different smoking transition pathways is limited. This study examined short-term associations between transitions to exclusive e-cigarette use, dual use, or cessation and physical health, mental health, and health-related quality of life, compared with continued smoking. MethodsA target trial emulation framework was applied to Waves 7-14 (2015-2024) of the UK Household Longitudinal Study, including 18,011 participant-wave observations from baseline smokers. Propensity score matching (1:3) was used to create comparable exposure groups. A doubly robust analysis-combining matching with Ordinary Least Squares regression-estimated outcomes using the SF-12 Physical (PCS) and Mental (MCS) Component Summary scores and a mapped EuroQol 5-Dimensions 3-Level version (EQ-5D-3L) index. The SF-12 is a validated generic health measure, where PCS and MCS are norm-based scores (mean = 50, SD = 10). The EQ-5D-3L index (range: 0 to 1) reflects overall health utility. ResultsCompared with continued smokers, exclusive e-cigarette users had higher short-term mental health scores (SF-12 MCS {beta} = 1.042; 95% CI: 0.229 to 1.855). In contrast, dual users had lower mental health scores ({beta} = -1.023; 95% CI: -1.574 to -0.472). Short-term physical health scores (SF-12 PCS) were lower among both exclusive switchers ({beta} = -0.670; 95% CI: - 1.287 to -0.053) and quitters ({beta} = -0.486; 95% CI: -0.853 to -0.119), with no evidence of short-term physical health improvement for any transition group. Dual users also had lower health-related quality of life (EQ-5D-3L {beta} = -0.016; 95% CI: -0.025 to -0.008). Subgroup analyses suggested heterogeneity by age and socioeconomic position, with poorer outcomes among older and more disadvantaged smokers. Sensitivity analyses produced directionally consistent findings. ConclusionExclusive switching to e-cigarettes was associated with higher short-term mental health scores, whereas dual use was associated with poorer mental health and health-related quality of life. These findings underscore the importance of distinguishing complete switching from dual use when designing harm-reduction policies and smoking cessation support.