Abstract: We identified 37 eligible studies involving over 15,000 participants who smoked tobacco. The majority of studies recruited participants with particular characteristics, often from groups of people who are less likely to seek support to stop smoking than the general population. Although a few studies recruited participants who intended to stop smoking soon or had no intentions to quit, most recruited a population without regard to their intention to quit. MI was conducted in one to 12 sessions, with the total duration of MI ranging from five to 315 minutes across studies. We judged four of the 37 studies to be at low risk of bias, and 11 to be at high risk, but restricting the analysis only to those studies at low or unclear risk did not significantly alter results, apart from in one case - our analysis comparing higher to lower intensity MI. We found low-certainty evidence, limited by risk of bias and imprecision, comparing the effect of MI to no treatment for smoking cessation (RR = 0.84, 95% CI 0.63 to 1.12; I2 = 0%; adjusted N = 684). One study was excluded from this analysis as the participants recruited (incarcerated men) were not comparable to the other participants included in the analysis, resulting in substantial statistical heterogeneity when all studies were pooled (I2 = 87%). Enhancing existing smoking cessation support with additional MI, compared with existing support alone, gave an RR of 1.07 (95% CI 0.85 to 1.36; adjusted N = 4167; I2 = 47%), and MI compared with other forms of smoking cessation support gave an RR of 1.24 (95% CI 0.91 to 1.69; I2 = 54%; N = 5192). We judged both of these estimates to be of low certainty due to heterogeneity and imprecision. Low-certainty evidence detected a benefit of higher intensity MI when compared with lower intensity MI (RR 1.23, 95% CI 1.11 to 1.37; adjusted N = 5620; I2 = 0%). The evidence was limited because three of the five studies in this comparison were at risk of bias. Excluding them gave an RR of 1.00 (95% CI 0.65 to 1.54; I2 = n/a; N = 482), changing the interpretation of the results. Mental health and quality of life outcomes were reported in only one study, providing little evidence on whether MI improves mental well-being. There is insufficient evidence to show whether or not MI helps people to stop smoking compared with no intervention, as an addition to other types of behavioural support for smoking cessation, or compared with other types of behavioural support for smoking cessation. It is also unclear whether more intensive MI is more effective than less intensive MI. All estimates of treatment effect were of low certainty because of concerns about bias in the trials, imprecision and inconsistency. Consequently, future trials are likely to change these conclusions. There is almost no evidence on whether MI for smoking cessation improves mental well-being.

Plain-language summary: This review included 37 trials covering over 15,000 people who smoked tobacco. Studies were conducted in a lot of different types of people, including people with health problems or drug use problems, young people, homeless people, and people who had been arrested or were in prison. Some people felt ready to quit smoking and others did not. Motivational interviewing was provided in one to 12 sessions and took from as little as five minutes, to as much as eight hours, to deliver. Studies lasted for at least six months. The evidence is up to date to August 2018. There was not enough information available to decide whether motivational interviewing helped more people to stop smoking than no stop smoking treatment. People were slightly more likely to stop smoking if they were provided with motivational interviewing rather than another type of treatment to stop smoking, but our findings suggest that there is still a chance that motivational interviewing could also reduce a person's chances of quitting compared with other stop smoking treatments. This means more research is needed to decide whether motivational interviewing can help more people to quit than other types of treatment. Using longer motivational interviewing with more treatment sessions may help more people to give up smoking than shorter motivational interviewing with fewer sessions, however more research is needed to be sure that this is the case. We also looked at whether being provided with motivational interviewing to quit smoking increased people's well-being. Most studies did not provide any information about this, and so more studies are needed to answer this question. There is low-quality evidence looking at whether motivational interviewing helps more people to quit smoking than no treatment. This means it is difficult to know whether motivational interviewing helps people to quit smoking or not, and more studies are needed. The quality of the evidence was also low for all of the other questions we asked about quitting smoking, which means that our findings may change when new research is carried out. The quality of the research is rated as low because there were problems with the design of studies, findings of studies were very different to one another, and there were not enough data, making it difficult to determine whether motivational interviewing or more intense motivational interviewing helped people to quit smoking or not.