Background:
The use of electronic cigarettes (e-cigarettes) has increased dramatically over the past decade. Although the majority of users report initiating e-cigarette use (vaping) to quit or reduce use of combustible cigarettes (smoking), a substantial proportion become “dual users” of both. Little is known about the patterns of dual use of, and dependence on, these products. This study describes those patterns as well as associations between smoking with vaping variables.

Methods:
During 2016-17, 2894 dual users were recruited into a 2-year longitudinal study testing nicotine cessation interventions. Inclusion criteria required at least weekly use of both products. This report uses baseline questionnaire data collected via mail or Internet before randomization. Descriptive and inferential statistics were calculated.

Results:
Participants' mean age was 29.9 (SD 11.3) years, and 63% were male. They reported smoking a median of 16-20 cigarettes per day (CPD) for a mean of 13 years before vaping, which was initiated a median of 13-24 months earlier. Smoking declined to 6-10 CPD currently, with 79% reporting decreased and 3% reporting increased smoking. 69% reported daily vaping, with 57% reporting vaping “continuously” or at least 30 times/day. CPD was inversely related to both weekly (days) and daily (uses) vaping frequency (ps < .001), but positively associated with nicotine dosage of the e-cigarette solution (p < .001). Duration of vaping was not associated with cigarette use (CPD, time to first cigarette) or dependence, but was positively associated with a measure of e-cigarette dependence (p < .001). Additional results will be reported.

Conclusions:
The current cross-sectional findings suggest that vaping may reduce and partially replace cigarette use. However, e-cigarette dependence may increase over time without further reduction in smoking among those who maintain dual use. Future results from this longitudinal study should clarify use patterns, with implications for both tobacco intervention and policy.