Table 2.

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

Author, year	Population	No.* of participants intend to quit smoking	No.* of participants in intervention or exposure groups and comparator	Description of intervention or exposure groups	Description of comparators	Measured outcomes	Definition of quitters or abstinence
Randomized controlled trials
Adriaens, 201433	Participants 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”) ENDS 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 quitting	No more cigarette smoking
Bullen, 201334–39	Had smoked ten or more cigarettes per day for the past year, interested in quitting	Yes 657 No 0	ENDS: 289 NRT: 295 ENNDS: 73	16 mg nicotine ENDS	21 mg patches NRT ENNDS	Continuous smoking abstinence, biochemically verified (eCO measurement <10 ppm); seven day point prevalence abstinence; reduction; and adverse events	Abstinence allowing ≤5 cigarettes in total, and proportion reporting no smoking of tobacco cigarettes, not a puff, in the past 7 days
Caponnetto, 201325	Smokers not intending to quit	Yes 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	ENNDS	Self-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 events	Complete self-reported abstinence from tobacco smoking - not even a puff
Cohort studies
Al-Delaimy, 201540	Current smokers; regardless of whether the users were using ENDS as part of a quit attempt	Yes 415 No 542	ENDS: 236Ψ No ENDS: 392Ψ	ENDS	No ENDS	Quit attempts; 20% reduction in monthly no. of cigarettes; and current abstinence from cigarette use	Duration of abstinence of one month or longer to be currently abstinent
Biener, 201529	All respondents had reported being cigarette smokers at baseline; regardless of whether the users were using ENDS as part of a quit attempt	Yes 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 specified	Smoking cessation was defined as abstinence from cigarettes for at least one month
Brose, 201526–28	Current smokers; regardless of whether the users were using ENDS as part of a quit attempt	Not reported	ENDS: 1507 No ENDS: 2610	ENDS daily ENDS non-daily	No ENDS€	Quit attemptsφ; cessationϖ; and substantial reduction defined as a reduction by at least 50% from baseline CPD to follow-up CPD	Change from being a smoker at baseline to being an ex-smoker at follow-up was coded as cessation
Hajek, 201546	69% (n=69) accepted e-cigs as part of their smoking cessation treatment	Not reported	ENDS: 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 ENDS	Self-reported abstinence was biochemically validated by exhaled CO levels in end-expired breath using a cut-off point on 9ppm, adverse events	Self-reported abstinence from cigarettes at 4 weeks
Harrington, 201545	Hospitalized cigarette smokers. All were cigarette smokers initially; regardless of whether the users were using ENDS as part of a quit attempt	Yes: 220*** No: not reported	ENDS: 171 No ENDS: 759	ENDS	No ENDS	Quitting smoking based on 30-day point prevalence at 6 months	Only self-reported quitting smoking
Manzoli, 201542	Smokers 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 reported	ENDS: 343 Tobacco and ENDS: 319 Tobacco only: 693	ENDS Tobacco and ENDS	Tobacco cigarettes only	Abstinence, proportion of quitters, biochemically verified (eCO measurement>7ppm), reduce tobacco smoking, and serious adverse events	Percentage of subjects reporting sustained (30 days) smoking abstinence from tobacco smoking
Borderud, 201441	Patients 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 attempt	Yes 633¥ No 42¥	ENDS: 285 No ENDS: 789	ENDS£+Evidence-based behavioral and pharmacologic treatment	No ENDS+Evidence-based behavioral and pharmacologic treatment	Smoking cessation by self-report	Patients were asked if they had smoked even a puff of a (traditional) cigarette within the last 7 days
Prochaska, 201443	Adult daily smokers with serious mental illness; regardless of whether the users were using ENDS as part of a quit attempt	At baseline, 24% intended to quit smoking in the next month	ENDS: 101 No ENDS: 855	ENDS	No ENDS	Smoking cessation by self-report and, biochemically verified (CO and cotinine)	Past 7 day tobacco abstinence
Vickerman, 201344	Adult tobacco current or past users; regardless of whether the users were using ENDS as part of a quit attempt	Not reported	ENDS: 765 No ENDS: 1,711	ENDS used for 1 month or more ENDS used for less than 1 month	No ENDS (never tried)	Tobacco abstinence	Self-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.