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Copyright © 2020 The Cochrane Collaboration. Published by John Wiley & Sons, Ltd. BACKGROUND: Populations experiencing homelessness have high rates of tobacco use and experience substantial barriers to cessation. Tobacco-caused conditions are among the leading causes of morbidity and mortality among people experiencing homelessness, highlighting an urgent need for interventions to reduce the burden of tobacco use in this population. OBJECTIVES: To assess whether interventions designed to improve access to tobacco cessation interventions for adults experiencing homelessness lead to increased numbers engaging in or receiving treatment, and whether interventions designed to help adults experiencing homelessness to quit tobacco lead to increased tobacco abstinence. To also assess whether tobacco cessation interventions for adults experiencing homelessness affect substance use and mental health. SEARCH METHODS: We searched the Cochrane Tobacco Addiction Group Specialized Register, MEDLINE, Embase and PsycINFO for studies using the terms: un-housed*, homeless*, housing instability, smoking cessation, tobacco use disorder, smokeless tobacco. We also searched trial registries to identify unpublished studies. Date of the most recent search: 06 January 2020. SELECTION CRITERIA: We included randomized controlled trials that recruited people experiencing homelessness who used tobacco, and investigated interventions focused on the following: 1) improving access to relevant support services; 2) increasing motivation to quit tobacco use; 3) helping people to achieve abstinence, including but not limited to behavioral support, tobacco cessation pharmacotherapies, contingency management, and text- or app-based interventions; or 4) encouraging transitions to long-term nicotine use that did not involve tobacco. Eligible comparators included no intervention, usual care (as defined by the studies), or another form of active intervention. DATA COLLECTION AND ANALYSIS: We followed standard Cochrane methods. Tobacco cessation was measured at the longest time point for each study, on an intention-to-treat basis, using the most rigorous definition available. We calculated risk ratios (RRs) and 95% confidence intervals (CIs) for smoking cessation for each study where possible. We grouped eligible studies according to the type of comparison (contingent reinforcement in addition to usual smoking cessation care; more versus less intensive smoking cessation interventions; and multi-issue support versus smoking cessation support only), and carried out meta-analyses where appropriate, using a Mantel-Haenszel random-effects model. We also extracted data on quit attempts, effects on mental and substance-use severity, and meta-analyzed these outcomes where sufficient data were available. MAIN RESULTS: We identified 10 studies involving 1634 participants who smoked combustible tobacco at enrolment. One of the studies was ongoing. Most of the trials included participants who were recruited from community-based sites such as shelters, and three included participants who were recruited from clinics. We judged three studies to be at high risk of bias in one or more domains. We identified low-certainty evidence, limited by imprecision, that contingent reinforcement (rewards for successful smoking cessation) plus usual smoking cessation care was not more effective than usual care alone in promoting abstinence (RR 0.67, 95% CI 0.16 to 2.77; 1 trial, 70 participants). We identified very low-certainty evidence, limited by risk of bias and imprecision, that more intensive behavioral smoking cessation support was more effective than brief intervention in promoting abstinence at six-month follow-up (RR 1.64, 95% CI 1.01 to 2.69; 3 trials, 657 participants; I2 = 0%). There was low-certainty evidence, limited by bias and imprecision, that multi-issue support (cessation support that also encompassed help to deal with other challenges or addictions) was not superior to targeted smoking cessation support in promoting abstinence (RR 0.95, 95% CI 0.35 to 2.61; 2 trials, 146 participants; I2 = 25%). More data on these types of interventions are likely to change our interpretation of these data. Single studies that examined the effects of text-messaging support, e-cigarettes, or cognitive behavioral therapy for smoking cessation provided inconclusive results. Data on secondary outcomes, including mental health and substance use severity, were too sparse to draw any meaningful conclusions on whether there were clinically-relevant differences. We did not identify any studies that explicitly assessed interventions to increase access to tobacco cessation care; we were therefore unable to assess our secondary outcome 'number of participants receiving treatment'. AUTHORS' CONCLUSIONS: There is insufficient evidence to assess the effects of any tobacco cessation interventions specifically in people experiencing homelessness. Although there was some evidence to suggest a modest benefit of more intensive behavioral smoking cessation interventions when compared to less intensive interventions, our certainty in this evidence was very low, meaning that further research could either strengthen or weaken this effect. There is insufficient evidence to assess whether the provision of tobacco cessation support and its effects on quit attempts has any effect on the mental health or other substance-use outcomes of people experiencing homelessness. Although there is no reason to believe that standard tobacco cessation treatments work any differently in people experiencing homelessness than in the general population, these findings highlight a need for high-quality studies that address additional ways to engage and support people experiencing homelessness, in the context of the daily challenges they face. These studies should have adequate power and put effort into retaining participants for long-term follow-up of at least six months. Studies should also explore interventions that increase access to cessation services, and address the social and environmental influences of tobacco use among people experiencing homelessness. Finally, studies should explore the impact of tobacco cessation on mental health and substance-use outcomes.

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The Cochrane database of systematic reviews

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