["Addiction, EarlyView. ", "\nAbstract\n\nBackground and aims\nDual use of cigarettes and e‐cigarettes is often employed by smokers as a harm reduction strategy. This study evaluated the harm reduction potential of e‐cigarettes by investigating changes in urinary biomarker of exposure (BOE) to tobacco‐specific nitrosamines (TSNAs) and nicotine among smokers transitioning to dual use.\n\n\nMethods\nData from 8688 adult smokers in waves 1 to 5 of the Population Assessment of Tobacco and Health Study (United States) were analyzed. ‘Transitioned dual users’ were defined as those changing from exclusive cigarette smoking to dual use across two consecutive waves. BOE levels were assessed before and after this transition and compared with those who continued smoking cigarettes exclusively (‘remained smokers’) across the same time frame. Propensity score weighting was used to adjust for baseline confounders. Further analyses were stratified by baseline cigarette consumption, dichotomized at the cohort median into heavy and light smoking groups.\n\n\nResults\nObservational analysis revealed that transitioned dual users experienced a statistically significant reduction in TSNAs without a statistically significant change in nicotine. The regression analysis comparing transitioned dual users with ‘remained smokers’ showed that transitioning to dual use was statistically significantly associated with a 13% decrease in 4‐(methylnitrosamino)‐1‐(3‐pyridyl)‐1‐butanol (NNAL) [95% confidence interval (CI) = −20% to −6%] and a 10% decrease in N′‐nitrosonornicotine (NNNT) (95% CI = −15% to −4%). In contrast, dual use was statistically significantly associated with a 17% increase in total nicotine equivalents‐2 (95% CI = 12%–23%) and 8% increase in total nicotine equivalents‐6 (95% CI = 5%–12%). Stratified analysis further showed that the association between dual use and TSNAs reduction was attenuated among the baseline heavy smoking group (>13 cigarettes/day), with one‐fourth of the effect estimates and no statistical significance.\n\n\nConclusions\nSmokers transitioning to dual use alongside e‐cigarettes appear to show reductions in tobacco‐specific nitrosamines exposure, particularly among lighter smokers, but do not appear to show reductions in nicotine exposure.\n\n"]