Background: There is a knowledge gap in the short- and long-term tobacco use patterns of adults who smoke cigarettes and initiate use of electronic cigarettes. This longitudinal study describes concurrent use transitions following e-cigarette initiation among adults who smoked cigarettes. Methods: From December 2020-October 2021, US adults who smoked cigarettes and recently (re-)initiated e-cigarette use within the past month (N = 303) were recruited nationally for a one-year online study consisting of weekly surveys for three months followed by three quarterly surveys. At each survey, current e-cigarette use, smoking status, and frequency of use were assessed to define their concurrent use state: neither vaping nor smoking, vaping only, predominantly vaping, equal dual use, predominantly smoking, or only smoking. Results: During the first 3 months, 74 % of participants shifted their usage states at least once, while 29 % shifted between 9 months and 12 months following e-cigarette initiation. Participants predominantly smoking initially tended to either continue predominantly smoking or returned to exclusively smoking. Those predominantly vaping tended to remain in this state or shift to only vaping. Individuals using both products split early between continued dual use, predominantly vaping, and predominantly smoking. Those only vaping tended to continue, although some reverted to smoking. Overall, initial concurrent use states appear somewhat predictive of trajectories and 12-month outcomes. Conclusions: Additional research on predictors (e.g., demographics, product characteristics, use experiences, perceptions and affect, product availability) of these frequent shifts between concurrent use states on weekly, monthly, and quarterly basis and 12-month outcomes is needed.
