Daily and Nondaily E-Cigarette Use in Relation to Cigarette Consumption Among Current Smokers.

backgroundDual use of e-cigarettes and conventional cigarettes is the most common pattern of e-cigarette use among adults who smoke. A central unresolved question is whether e-cigarettes partially displace cigarette consumption among dual users or supplement largely unchanged smoking, and whether any difference depends on the frequency of e-cigarette use. Population-based evidence examining smoking intensity and stratifying dual users by frequency of e-cigarette use remains limited.

methodsA pooled cross-sectional analysis was conducted using five waves (2017, 2018, 2019, 2021 and 2022) of the Scottish Health Survey, a nationally representative household survey. The analytic sample comprised 3150 current cigarette smokers aged 16 years and over, including 2605 cigarette-only smokers and 545 dual users of cigarettes and e-cigarettes. Two outcomes were examined: average number of cigarettes smoked per day and smoking intensity classified as light (< 10/day), moderate (10 to < 20/day) or heavy (≥ 20/day). Associations were estimated using multivariable linear regression, generalised ordered logit models and quantile regression, adjusting for age, sex, area deprivation, ethnicity, educational attainment and survey year. An additional analysis stratified dual users by frequency of e-cigarette use as daily, weekly nondaily or less-than-weekly users. Postestimation contrasts compared the frequency groups directly.

resultsDual users smoked fewer cigarettes per day than cigarette-only smokers (mean: 11.28 [SD: 7.07] vs. 12.65 [SD: 7.94]; p = 0.0002). After full adjustment, this corresponded to 1.10 fewer cigarettes per day (β = -1.10; 95% CI: -1.73 to -0.47; p = 0.001). Dual users also had 20% lower odds of being in a higher smoking-intensity category (OR = 0.80; 95% CI: 0.67 to 0.96; p = 0.014). The difference in cigarette consumption was larger at the upper end of the distribution, reaching approximately two fewer cigarettes per day at the 90th percentile (β = -1.99; 95% CI: -3.32 to -0.65; p = 0.004). In frequency-specific analyses, only daily e-cigarette users differed from cigarette-only smokers. Daily e-cigarette users smoked a mean of 9.90 cigarettes per day compared with 12.65 among cigarette-only smokers (adjusted β = -2.46; 95% CI: -3.32 to -1.59; p < 0.001) and had lower odds of being in a higher smoking-intensity category (OR = 0.57; 95% CI: 0.44 to 0.74; p < 0.001). Neither less-than-weekly nor weekly nondaily e-cigarette use was associated with lower cigarette consumption. Formal contrasts showed that daily e-cigarette users smoked 2.21 fewer cigarettes per day than less-than-weekly users (p = 0.010) and 2.23 fewer than weekly nondaily users (p = 0.001).

conclusionsConcurrent e-cigarette use was associated with modestly lower cigarette consumption among current smokers, but only among daily e-cigarette users, who still smoked about 10 cigarettes per day. This pattern may reflect partial cigarette displacement, selection or reverse causation, whereas nondaily e-cigarette use was more consistent with supplementary use. Dual use should therefore be viewed as a potentially transitional pattern rather than a harm-reduction endpoint.