BACKGROUND: Smoking rates are up to four times higher among people experiencing homelessness compared to the housed population and how to support them to quit is unclear.

OBJECTIVES: To test the effectiveness and cost-effectiveness of providing e-cigarettes to people accessing homeless support services to stop smoking. To explore implementation fidelity, contextual influences and mechanisms of change.

TRIAL DESIGN: A two-arm cluster randomised controlled trial with cost-effectiveness and mixed-methods process evaluation.

METHODS: Setting and participants: Thirty-two homeless centres (clusters) across Great Britain. Participants were aged 18+ years and known by centre staff to smoke. Sixteen staff and 31 participants in electronic cigarette centres completed qualitative interviews.

INTERVENTIONS: Clusters were randomised (1 : 1) to electronic cigarettes or usual care prior to staff training. Electronic cigarette participants received a refillable electronic cigarette, 4 weeks' supply of e-liquid and a fact sheet. Usual care participants received very brief advice on smoking, a support leaflet and signposting to Stop Smoking Service.

MAIN OUTCOME MEASURES: Primary outcome: Carbon-monoxide-verified sustained smoking abstinence from 2 weeks post baseline to 24 weeks. Secondary outcomes: Carbon-monoxide-verified 7-day point prevalence abstinence, < 50% smoking reduction, intervention costs, quality-adjusted life-years, implementation fidelity and mechanisms of change.

RESULTS: Sixteen centres were randomised to electronic cigarettes ( = 239 participants) and 16 to usual care ( = 238 participants). In usual care, one participant died, and one withdrew consent. Final sample analysed:  = 239 (electronic cigarette);  = 236 (usual care). Sustained 24-week carbon-monoxide-verified smoking cessation rates were 5/239 (2.1%) with electronic cigarettes versus 2/236 (0.8%) with usual care (adjusted risk ratio 2.43, 95% confidence interval 0.51 to 11.64); 7-day point prevalence abstinence at 24 weeks was 15/239 (6.3%) with electronic cigarettes versus 5/236 (2.1%) with usual care (adjusted risk ratio 2.95, 95% confidence interval 1.05 to 8.29). Smoking reduction (< 50%) at 24 weeks was 83/239 (34.7%) with electronic cigarettes versus 40/236 (17%) with usual care (adjusted risk ratio 2.02, 95% confidence interval 1.44 to 2.84). Four adverse events were reported in the electronic cigarette arm; three electronic cigarette-related and not serious; one serious and not electronic cigarette-related. Mean per participant intervention costs were £92 (standard error £0) for electronic cigarettes and £50 (standard error £0) for usual care. Mean total costs were £3859 (standard error £441) versus £2716 (standard error £386) in electronic cigarette and usual care groups, respectively. Quality-adjusted life-years were estimated at 0.303 (standard error 0.008) for electronic cigarettes and 0.295 (standard error 0.010) for usual care. From National Health Service and Personal Social Services perspective, the electronic cigarette arm was £1267 (95% confidence interval £219 to £2347) more costly and yielded 0.007 (95% confidence interval -0.016 to 0.033) more quality-adjusted life-years than usual care. The incremental cost-effectiveness ratio was £181,000 per quality-adjusted life-year gain. Electronic cigarette intervention was delivered with high fidelity with no variation by centre size or location, although staff capacity and resourcing were barriers to implementation. Quit enablers included high capability to use electronic cigarettes, support from friends/family and increased motivation following smoking reduction. Barriers included lack of satisfaction with electronic cigarettes compared with smoking, a strong smoking culture at centres and staff deprioritising tobacco-related harm.

LIMITATIONS: Low quit rates resulted in insufficient power to detect an effect on the primary effectiveness and cost-effectiveness outcomes, leading to uncertainty.

CONCLUSIONS AND FUTURE WORK: Electronic cigarettes showed no effectiveness for 24 weeks' sustained smoking abstinence; 7-day point prevalence abstinence and smoking reduction were possible, and staff can support the intervention. Longer-term and potentially different types of support and systems change are needed to support sustained abstinence.