Although RCTs showed that more people quit smoking with electronic cigarettes (ecigs) than with placebo, the number who quit was low in both groups and the evidence was of low certainty (94 vs 41 per 1000 people; all results on average). Low‐certainty evidence suggests the most common adverse events were cough, dry mouth, shortness of breath, throat irritation, and headache, with little difference between groups, and these were mostly reduced in both groups over time. One RCT showed little difference in absolute numbers of people who quit smoking or experienced an adverse event. Cohort studies reported variable smoking cessation rates from 5% to 46%, and confirmed that cough, dry/irritated mouth/throat, and headache were commonly reported adverse effects of ecigs, but all were underpowered. Only one cohort study assessed ecig use beyond 12 months, so the safety of long‐term ecig use remains uncertain.