Table 2

Study characteristics related to population, intervention or exposure groups, comparator, and assessed outcomes.

Author, yearPopulationNo.* of participants intend to quit smokingNo.* of participants in intervention or exposure groups and comparatorDescription of intervention or exposure groupsDescription of comparatorsMeasured outcomesDefinition of quitters or abstinence
Randomized controlled trials
Adriaens, 201433Participants unwilling to quit smoking (participants from the control group kept on smoking regular tobacco cigarettes during the first eight weeks of the study)Yes 0
No 50
ENDS 1: 16
ENDS 2: 17
Control/ENDS: 17
ENDS (“Joyetech eGo-C”)
E-cigarettes (“Kanger T2-CC”)
ENDS and e-liquid**Quitting, defined as eCO of 5 ppm or smaller; questionnaire self-report of reduction in cigarettes of>50% or complete quittingNo more cigarette smoking
Bullen, 201334–39Had smoked ten or more cigarettes per day for the past year, interested in quittingYes 657
No 0
ENDS: 289
NRT: 295
16 mg nicotine ENDS21 mg patches NRT
Continuous smoking abstinence, biochemically verified (eCO measurement <10 ppm); seven day point prevalence abstinence; reduction; and adverse eventsAbstinence allowing ≤5 cigarettes in total, and proportion reporting no smoking of tobacco cigarettes, not a puff, in the past 7 days
Caponnetto, 201325Smokers not intending to quitYes 0
No 300
ENDS 1: 100
ENDS 2: 100
ENNDS: 100
7.2 mg nicotine ENDS
7.2 mg nicotine ENDS+5.4 mg nicotine ENDS
ENNDSSelf-report of reduction in cigarettes of>50%; abstinence from smoking, defined as complete self-reported abstinence from tobacco smoking - not even a puff, biochemically verified (eCO measurement ≤7 ppm); and adverse eventsComplete self-reported abstinence from tobacco smoking - not even a puff
Cohort studies
Al-Delaimy, 201540Current smokers; regardless of whether the users were using ENDS as part of a quit attemptYes 415
No 542
ENDS: 236Ψ
No ENDS: 392Ψ
ENDSNo ENDSQuit attempts; 20% reduction in monthly no. of cigarettes; and current abstinence from cigarette useDuration of abstinence of one month or longer to be currently abstinent
Biener, 201529All respondents had reported being cigarette smokers at baseline; regardless of whether the users were using ENDS as part of a quit attemptYes 364β
No 331
1374$ENDS£ intermittent use
ENDS£ intensive use
No ENDS (used once or twice ENDS)Smoking cessation; and reduction in motivation to quit smoking among those who had not quit, not otherwise specifiedSmoking cessation was defined as abstinence from cigarettes for at least one month
Brose, 201526–28Current smokers; regardless of whether the users were using ENDS as part of a quit attemptNot reportedENDS: 1507
No ENDS: 2610
ENDS daily
ENDS non-daily
No ENDSQuit attemptsφ; cessationϖ; and substantial reduction defined as a reduction by at least 50% from baseline CPD to follow-up CPDChange from being a smoker at baseline to being an ex-smoker at follow-up was coded as cessation
Hajek, 20154669% (n=69) accepted e-cigs as part of their smoking cessation treatmentNot reportedENDS: 69
No ENDS: 31
ENDS was offered to all smokers in addition to the standard treatment (weekly support and stop smoking medications including NRT and varenicline)No ENDSSelf-reported abstinence was biochemically validated by exhaled CO levels in end-expired breath using a cut-off point on 9ppm, adverse eventsSelf-reported abstinence from cigarettes at 4 weeks
Harrington, 201545Hospitalized cigarette smokers. All were cigarette smokers initially; regardless of whether the users were using ENDS as part of a quit attemptYes: 220***
No: not reported
ENDS: 171
No ENDS: 759
ENDSNo ENDSQuitting smoking based on 30-day point prevalence at 6 monthsOnly self-reported quitting smoking
Manzoli, 201542Smokers of ≥1 tobacco cigarette/day (tobacco smokers), users of any type of e-cig, inhaling ≥50 puffs weekly (e-smokers), or smokers of both tobacco and e-cig (dual smokers)Not reportedENDS: 343
Tobacco and ENDS: 319
Tobacco only: 693
Tobacco and ENDS
Tobacco cigarettes onlyAbstinence, proportion of quitters, biochemically verified (eCO measurement>7ppm), reduce tobacco smoking, and serious adverse eventsPercentage of subjects reporting sustained (30 days) smoking abstinence from tobacco smoking
Borderud, 201441Patients who presented for cancer treatment and identified as current smokers (any tobacco use within the past 30 days); regardless of whether the users were using ENDS as part of a quit attemptYes 633¥
No 42¥
ENDS: 285
No ENDS: 789
ENDS£+Evidence-based behavioral and pharmacologic treatmentNo ENDS+Evidence-based behavioral and pharmacologic treatmentSmoking cessation by self-reportPatients were asked if they had smoked even a puff of a (traditional) cigarette within the last 7 days
Prochaska, 201443Adult daily smokers with serious mental illness; regardless of whether the users were using ENDS as part of a quit attemptAt baseline, 24% intended to quit smoking in the next monthENDS: 101
No ENDS: 855
ENDSNo ENDSSmoking cessation by self-report and, biochemically verified (CO and cotinine)Past 7 day tobacco abstinence
Vickerman, 201344Adult tobacco current or past users; regardless of whether the users were using ENDS as part of a quit attemptNot reportedENDS: 765
No ENDS: 1,711
ENDS used for 1 month or more
ENDS used for less than 1 month
No ENDS (never tried)Tobacco abstinenceSelf-reported 30-day tobacco abstinence at 7 month follow-up
  • no.: number; C: comparator group; CPD: cigarettes smoked per day; e-cig: e-cigarettes; ENDS: Electronic nicotine delivery system; ENNDS: electronic non-nicotine delivery systems; eCO: exhaled breath carbon monoxide; NE: non-exposure group; NRT: Nicotine replacement therapy.

  • *Numbers randomized or at baseline.

  • **For the first two months control group consisted of no e-cigarettes use. After that period, the participants of control group received the e-cig and e-liquid. ENDS1=“Joyetech eGo-C” e-cig and ENDS2=“Kanger T2-CC” e-cig.

  • ***Only among those who reported any previous use of e-cigs.

  • αInformation retrieved through contact with author.

  • The comparator comprises of current non-users of e-cig, which included never-users and those who had previously tried but were not using at the moment.

  • ΨParticipants who will never use e-cig plus those who never heard of e-cig=392; participants who have used e-cig=236 (numbers taken from the California Smokers Cohort, a longitudinal survey).

  • βIntentions to quit smoking, those who tried e-cigarettes only once or twice are grouped with never users (“non-users/triers”).

  • Intermittent use (i.e., used regularly, but not daily for more than 1 month) plus intensive use (i.e., used e-cig daily for at least 1 month).

  • $No. of the whole sample including comparator.

  • £All ENDS.

  • ¥The other participants either quit more than a month ago but less than six months, less than a month ago, or more than six months ago.

  • φSmokers and recent ex-smokers were asked about the number of attempts to stop they had made in the previous year. Those reporting at least one attempt and 37 respondents who did not report an attempt but had stopped smoking be- tween baseline and follow-up were coded as having made an attempt.

  • ϖChange from being a smoker at baseline to being an ex-smoker at follow-up was coded as cessation.