Table 1 Overview of studies included in the systematic review and meta-analysis (N = 16)

Balmford et al., 2010, Australia, USA, UK, Canada, [29]

Smokers or recent quitters who had used medication in the last year

Cross-sectional, 981

Participants provided with NRT for 8 weeks.

Participants were followed up to 6 months

Adherence is defined as the use of NRT for 8 weeks, with those who terminated before this cut point considered to have stopped prematurely. Successful smoking cessation was defined as continuous abstinence at six months.

Among the participants, 71.4% of NRT users discontinued medication use prematurely. Those who discontinued use prematurely were significantly less likely to achieve abstinence than those who completed the course of medication (OR = 0.16, 95% CI = 0.08–0.31).

Ben Taleb et al., 2015, Syria, [30]

Adult smokers 18 to 65 years old

RCT, 269

Participants provided with 6 weeks of nicotine patches.

Participants were followed up to 6 weeks

Participants were asked whether they had followed treatment instructions to use one patch every day over the past week. NRT adherence defined being adherent to patch use as responding “yes” to this question during at least 5 of the 6 weeks (> 80%).

Among participants on the nicotine patch, 68% were found to be adherent to pharmacological treatment.

Berg et al., 2013, USA, [31]

Adult smokers 18 years of age or above

RCT, 202

All participants received nicotine patches.

Participants were followed at 3, 6, and 12 months

Calculated adherence level as the number of patches used (80% adherence as adherent; <  80% adherence was considered nonadherent)

Among the study participants, 66.8% were adherent to the patch.

Bolliger et al., 2000, Switzerland, [32]

Adults 18 years of age and above

RCT, 400

Participants were provided with the inhaler as needed for up to 18 months.

Participants were followed at 1, 2, 3, and 6 weeks.

Adherence is defined as utilisation of inhaler every day. Smoking cessation is defined as a decrease in verified measurement of exhaled carbon monoxide at each time point compared with the measurement at baseline.

At week six, 222/368 (60%) participants used the inhaler every day. At 4, 12, and 18 months 146/318 (46%), 39/331 (12%), and 30/289 (10%) participants respectively used the inhaler every day.

Coleman et al., 2012, UK, [33]

Pregnant women GA 12–24 weeks smoked 5 or more cigarettes per day

RCT, 521

Participants were provided with 8 weeks of nicotine patches (15 mg per 16 h).

Participants were followed up to 8 weeks

Using the nicotine patches daily for at least 1 month.

Only 7.2% of women assigned to nicotine-replacement therapy used patches for more than 1 month

Fish et al., 2009, USA, [34]

Pregnant women, GA 13–25 weeks, smoked at least 100 cigarettes in their lifetime, currently smoking five or more per day

RCT, 104

6 weeks of NRT (choice of patch, gum, or lozenge) provided for the treatment arm. Women who chose the patch were provided with a 7-mg patch for fewer than 10 cigarettes/day, 14-mg patch for 10–14 cigarettes/day, and 21-mg patch for 15+ cigarettes/day. Gum or lozenge users were instructed to use one 2-mg piece for every cigarette smoked per day.

Participants were followed up to 6 weeks

Total days of nicotine patch use per week of NRT use over the 6 weeks treatment period.

29% of the 104 women used NRT for the recommended 6 weeks as directed by the health provider.

Hollands et al., 2013, UK, [21]

Adult smokers starting a quit attempt

RCT, 633

All participants were prescribed a nicotine patch and oral NRT. Those smoking 15 or more cigarettes daily were prescribed 21 mg/24 h patches and those smoking 10–14 cigarettes daily were prescribed 14 mg patches.

Participants were followed up to 4 weeks

The proportion of all NRT prescribed consumed each day, averaged over the 4-week treatment period. Quitting is defined as self-reported abstinence from smoking at 4 weeks.

Participants using > 80% of prescribed NRT over 4 weeks 83.8 (351). Higher consumption of NRT was associated with a nonsignificant increase in abstinence (p = .093 each additional mg/day consumed was associated with increased odds of abstinence of 5%. OR = 1.05 (95%CI, 1.01–1.10).

Hotham et al., 2006, Australia, [35]

Smoking at least 15 cigarettes per day, GA 12–28 weeks

RCT, 20

Participants offered nicotine patches (15 mg/16 h) for a maximum of 12 weeks.

Participants were followed up to 12 weeks

Self-reported nicotine patch use as directed by the health care provider for the duration of up to 12 weeks.

Only 5 (25%) women used patches continuously up to the 12-week maximum.

Kapur et al., 2001, Canada, [36]

Pregnant women GA 12–24 weeks, who smoke 15 or more cigarettes per day

RCT, 104

Participants were provided with 18 h patch of nicotine 15 mg for 8 weeks, 10 mg for an additional 2 weeks, and 5 mg for the last 2 weeks.

Participants were followed up to 12 weeks

Self-reported use of NRT as prescribed for the duration of up to 12 weeks.

Among participants, 4 (23.5%) women completed the whole course of prescribed medication

Lam et al., 2004, China, [8]

Adult current smokers

Cross-sectional, 1051

Participants are provided with NRT for 12 weeks.

Participants were followed up to 12 months

Self-reported use of NRT daily for at least 4 weeks during the first 3 months. Quitting is assessed by asking whether the subjects had smoked any cigarette during the past 7 days at the 12 months (point prevalence quit rate).

The prevalence of adherence (self-reported NRT use for at least 4 weeks) was 16% (95% confidence interval 14–18%). The quit rate in the adherent group (40%) was greater than that of the non-adherent group (25%) (P < 0.001).

Schneider et al., 2003, Switzerland, [22]

Adult daily smokers who are motivated to quit

Cross-sectional exploratory study, 82

Participants were provided with a nasal spray.

Participants were followed up to at week 2, and every month for 24 months

The pharmacist checked every spry thoroughly with a software program and compared the total record of puffs since the last visit with the weight of the returned nasal spray bottles during the first month. Self-reported continuous abstinence from smoking from the end of the first month to the end of the week 2 and at 1, 2, 3, 4, 6, 9, 12, 15, 18, 21, and 24 months of follow-up, validated by expired-air carbon monoxide.

Among the participants, 80% (29/36) of the failures were low consumers of the nasal spray (0–15 puffs/ day) compared with 54% (25/46) of the abstainers. Only 11% (5/46) of the abstainers and 3% (1/36) of the failures used the spray extensively during the first month of the study (i.e., more than 30 puffs/day).

Shiffman et al., 2008, USA, [37]

Adult current smokers

RCT, 204

A 6-week supply of nicotine patches was provided.

Participants were followed up to 6 weeks

Using the patch for ≥20 of the first 21 days of treatment. Smoking cessation is defined as self-reported abstinence at 6 weeks.

Rates of adherence did not differ significantly between the active and placebo groups (139 [68.1%] and 114 [68.3%], respectively). Among active patch users, the odds of abstinence at 6 weeks were more than 3 times greater for adherent versus non-adherent subjects (53.2% vs 21.5%, respectively; OR = 4.20; 95% CI, 1.51–11.72; P = 0.006), (P = 0.027).

Shiffman et al., 2006, USA, [10]

Adults who maintained abstinence for the first 2 weeks of treatment.

RCT, 1020

Participants were provided with lozenge and behavioural advice.

Participants were followed up to 6 weeks

Average daily lozenge use during the first 2 weeks of therapy. Smoking cessation assessed as a 28 days continuous abstinence, verified by carbon monoxide readings < 10 ppm.

The odds of smoking cessation were 1.25; CI = 1.05–1.50, P < 0.02) higher for the adherence group.

Voci et al., 2016, Canada, [38]

18 years or older who smoked at least 10 cigarettes daily

Cross-sectional, 1605

Participants were provided with NRT. The selection of types of NRT was based on participant preference.

Participants were followed up to 10 weeks

Self-reported amount of NRT used over 10 weeks period. The quit outcome was a 7-day point-prevalence of abstinence at a 6-month follow-up.

Of those with end-of-treatment follow-up data on amount of NRT used (n = 1605), 19.8% (n = 318) used all 10 weeks of NRT provided. Poor quit success was reported by those who used either some (AOR = 0.43, 95% CI = 0.26–0.69, p = 0.001) or none (AOR = 0.30, 95% CI = 0.09–0.95, p = 0.041) of the NRT versus all 10 weeks.

Wisborg et al., 2000, Denmark, [39]

Pregnant women who smoked 10 or more cigarettes after the first trimester

RCT, 124

Participants were provided with 15-mg patches (16 h/day) for 8 weeks, and/or 10-mg patches (16 h/day) for 3 weeks.

Participants were followed up to 8 weeks

The number of nicotine patches used in 8 weeks period. Quitting is defined as self-reported abstinence of at least 7 days at second, third, and fourth prenatal visits.

In the nicotine group, 17% used all nicotine patches as prescribed by the health care provider.

Yingst et al., 2015, USA, [40]

current daily smokers

Follow-up cross-sectional study, 201

Participants were provided with NRT for a 2-week supply (14 patches) of 21 mg/24-h nicotine patches at the first group visit.

Participants were followed up to 4 weeks

Adherence to the directed use of the nicotine patch was measured by the number of self-reported days, of 28 days, the patch was worn during the quit attempt in treatment. Participants were considered adherent if the patch was worn all 28 days and non-adherent if the nicotine patch was worn less than 28 days.

Among participants, 71 (35.3%) participants were adherent for the first 28 days of treatment and 130 (64.7%) participants were non-adherent.