Practical alcohol risk-reduction advice plus a brief commitment declaration in a social drinking laboratory: a pilot cluster randomized trial

BackgroundPractical alcohol risk-reduction strategies are widely recommended in public-facing alcohol guidance, but randomized evidence from socially interactive drinking episodes remains limited. We conducted a pilot cluster randomized trial to evaluate the feasibility and preliminary effects of a package intervention comprising practical drinking-strategy information, participant self-selection of same-day strategies, and a brief commitment declaration in a social drinking laboratory. MethodsThis single-center, parallel-group pilot trial was conducted in Japan. Pre-existing social groups participated. One or two groups scheduled in the same session slot were combined into a time-slot allocation unit, which was randomized 1:1 either to the package intervention or to alcohol-related knowledge only. The primary outcome was total pure alcohol intake during the first 120 min. Session satisfaction on a Visual Analog Scale (VAS) was a prespecified secondary participant-experience outcome. ResultsOf 83 interested individuals, 63 were randomized and 59 participants in 17 social groups and 12 allocation units were included in the modified intention-to-treat analysis. The mean paired intervention-control difference for 120-min alcohol intake was-8.84 g (95% confidence interval [CI]-27.92 to 10.23; exact sign-flip p = 0.281). The corresponding exploratory 0-30 min difference was-4.90 g (95% CI-10.48 to 0.68; exact sign-flip p = 0.094). In a genotype-adjusted participant-level sensitivity analysis, the intervention coefficient for 120-min intake was-16.0 g (95% CI-30.9 to-1.1; p = 0.036). Session satisfaction was high in both arms with no clear between-arm difference. Next-day follow-up was 100%, and no adverse-event-related discontinuations occurred. ConclusionsThe intervention was feasible to deliver in a socially interactive drinking setting, and session satisfaction was high in both arms. Primary allocation-unit estimates favored lower alcohol intake but were imprecise. Larger trials are needed to estimate effects more precisely, while considering the potential influence of genotype imbalance on effect estimation in East Asian samples. Trial registrationUniversity Hospital Medical Information Network Clinical Trials Registry (UMIN-CTR) UMIN000060685. Registered 17 February 2026.