Outcomes Critical outcomes were abstinence from smoking after at least six months, adverse events (AEs), and serious adverse events (SAEs). Important outcomes were biomarkers, toxicants/carcinogens, and longer‐term EC use.

Risk of bias

We used the RoB 1 tool to assess risk of bias for each study and GRADE to assess evidence certainty.

Synthesis methods

We followed standard Cochrane methods for screening and data extraction. Where appropriate, we pooled data using random‐effects models to calculate risk ratios (RRs) with 95% confidence intervals (CIs) for dichotomous outcomes. For continuous outcomes, we calculated mean differences with 95% CIs.

Included studies

We included 104 completed studies (14 new to this update), representing 30,366 participants, of which 61 were randomised controlled trials (RCTs). We rated 11 included studies as being at low risk of bias, 70 at high risk (including all non‐randomised studies), and the remainder at unclear risk overall.

Synthesis of results

Nicotine EC result in increased quit rates compared to nicotine replacement therapy (NRT) (high‐certainty evidence) (RR 1.55, 95% CI 1.28 to 1.88; I² = 0%; 9 studies, 2703 participants). In absolute terms, this might translate to an additional three quitters per 100 (95% CI 2 to 5 more). The rate of occurrence of AEs is probably similar between groups (moderate‐certainty evidence (limited by imprecision)) (RR 1.00, 95% CI 0.73 to 1.37; I² = 58%; 7 studies, 2241 participants). SAEs were rare, and there is insufficient evidence to determine whether rates differ between groups due to very serious imprecision (RR 1.22, 95% CI 0.73 to 2.03; I² = 30%; 8 studies, 2950 participants; low‐certainty evidence).

Nicotine EC probably result in increased quit rates compared to non‐nicotine EC (moderate‐certainty evidence, limited by imprecision) (RR 1.34, 95% CI 1.06 to 1.70; I² = 0%; 7 studies, 1918 participants). In absolute terms, this might lead to an additional two quitters per 100 (95% CI 0 to 4 more). There is probably little to no difference in the rate of AEs between these groups (moderate‐certainty evidence) (RR 1.01, 95% CI 0.95 to 1.08; I² = 0%; 5 studies, 840 participants). There is insufficient evidence to determine whether rates of SAEs differ between groups, due to very serious imprecision (RR 0.98, 95% CI 0.55 to 1.73; I² = 0%; 10 studies, 1717 participants; low‐certainty evidence).

Compared to behavioural support only or no support, quit rates may be higher for participants randomised to nicotine EC (low‐certainty evidence due to risk of bias) (RR 1.78, 95% CI 1.42 to 2.25; I² = 13%; 11 studies, 6819 participants). In absolute terms, this represents an additional three quitters per 100 (95% CI 2 to 5 more). There was some evidence that (non‐serious) AEs may be more common in people randomised to nicotine EC (RR 1.22, 95% CI 0.96 to 1.55; I² = 66%; 8 studies, 2485 participants; very low‐certainty evidence) but the evidence is uncertain and, again, there was insufficient evidence to determine whether rates of SAEs differed between groups (RR 0.93, 95% CI 0.67 to 1.29; I² = 0%; 15 studies, 4716 participants; very low‐certainty evidence).

Data from non‐randomised studies were consistent with RCT data. The most commonly reported AEs were throat/mouth irritation, headache, cough, and nausea, which tended to dissipate with continued EC use. Very few studies reported data on other outcomes or comparisons; hence, evidence for these is limited, with CIs often encompassing both clinically significant harm and benefit.