title: Prevalence and correlates of electronic-cigarette use in young adults: findings from three studies over five years.
abstract: BACKGROUND
  We aimed to examine prevalence and correlates of past-month electronic cigarette ("e-cigarette") use and use of e-cigarettes to aid a cessation attempt in three samples of young adult smokers recruited online in 2009-2010 (Study 1), 2010-2011 (Study 2), and 2013 (Study 3).
  
  
  METHODS
  Participants were young adults aged 18 to 25 who smoked at least one cigarette in the previous month (Study 1, N=1987 and Study 2, N=570) or smoked 3 or more days each week and used Facebook 4 or more days per week (Study 3, N=79). We examined both past-month e-cigarette use and ever use of e-cigarettes to quit conventional cigarettes.
  
  
  RESULTS
  Prevalence of past-month use of e-cigarettes was higher in each subsequent study: Study 1 (6%), Study 2 (19%), and Study 3 (41%). In multivariate analyses, significant correlates of past-month e-cigarette use were identified for Study 1 (male sex OR=2.1, p=.03; past-year quit attempt OR=1.6, p=.03) and Study 2 (male sex, OR=1.7, p=.03; younger age OR=0.88, p=.05), but not Study 3. In multivariate analyses, significant correlates of ever use of e-cigarette to quit conventional cigarettes were identified for Study 1 (education, OR=1.2, p=.02; smoking within 30min of waking, OR=2.8, p=.02; past year quit attempt OR=4.1, p=.02), and Study 3 (desire to quit smoking, OR=1.3, p=.02), but not Study 2.
  
  
  CONCLUSIONS
  E-cigarette use is increasingly common among young adults, particularly men. E-cigarette use for quitting conventional cigarettes appears more common among those more nicotine dependent and interested in quitting.
authors.affiliation: Department of Psychiatry, University of California, San Francisco, 401 Parnassus Avenue, Box TRC 0984, San Francisco, CA 94143, USA. Electronic address: danielle.ramo@ucsf.edu.
authors.forename: Danielle E
authors.initials: DE
authors.lastname: Ramo
authors.affiliation: Division of Research, Kaiser Permanente Northern California, 2000 Broadway-3rd floor, Oakland, CA 94612, USA. Electronic address: kelly.c.young-wolff@kp.org.
authors.forename: Kelly C
authors.initials: KC
authors.lastname: Young-Wolff
authors.affiliation: Stanford Prevention Research Center, Department of Medicine, Stanford University, Medical School Office Building, 1265 Welch Road, Stanford, CA 94305-5411, USA. Electronic address: jpro@stanford.edu.
authors.forename: Judith J
authors.initials: JJ
authors.lastname: Prochaska
mesh_terms: D000293:Adolescent; D000328:Adult; D000367:Age Factors; D066300:Electronic Nicotine Delivery Systems / Q000706:statistics & numerical data*; D005260:Female; D006801:Humans; D008297:Male; D009042:Motivation; D015995:Prevalence; D016540:Smoking Cessation / Q000379:methods / Q000706:statistics & numerical data; D014029:Tobacco Use Disorder / Q000453:epidemiology* / Q000628:therapy; D014481:United States / Q000453:epidemiology; D055815:Young Adult
keywords: E-cigarette; ENDS; Electronic cigarette; Smoking; Vaping; Young adults