Importance Preliminary studies suggest heart rate variability biofeedback (HRVB) may reduce craving and negative affect in individuals with substance use disorder (SUD), but few studies have evaluated whether this translates into improved substance use outcomes, and no prior studies have examined second-generation wearable HRVB technology in this context. Objective To evaluate the effects of second-generation HRVB on negative affect, positive affect, craving, and alcohol and other drug (AOD) use in adults with SUD. Design, Setting, and Participants This phase 2 randomized clinical trial included 8 weeks of outpatient treatment. Recruitment was conducted virtually across the US from February 2023 to June 2024. Treatment-seeking adults with SUD were randomized to receive HRVB + treatment as usual (TAU) or TAU only. Intervention Eight weeks of HRVB. Main Outcomes and Measures The primary outcomes were negative affect, positive affect, craving, and substance use, assessed with ecological momentary assessment. Results Of 260 individuals assessed for eligibility, 120 were randomized to receive HRVB + TAU or TAU only. Among study participants (69 female participants of 115 60.0%; mean SD age, 46.18 11.59 years), HRVB was associated with significant reductions in negative affect ( b , –0.01; z , –3.21; P = .001) and craving ( b , –0.01; z , –4.60; P lt; .001) over 8 weeks. In contrast, the control group experienced increases in both negative affect and craving. No differences were observed for positive affect. HRVB was also associated with a significantly lower proportion of AOD use days (odds ratio OR, 0.36; 95% credible interval CrI, 0.25-0.54), representing a 64% reduction in AOD use compared to controls. Treatment condition moderated the within-person relationship between craving and later AOD use (OR, 0.84; 95% CrI, 0.73-0.97), such that those receiving HRVB were less likely to use AOD following craving ( b , –0.18; 95% CrI, –0.32 to –0.03). Conclusions and Relevance In this randomized clinical trial, findings suggest second-generation HRVB can reduce negative affect, craving, and substance use among individuals in early recovery from SUD. HRVB appears to confer benefit in part by disrupting the association between craving and subsequent AOD use; these results support HRVB as a potentially efficacious treatment for SUD and warrant further investigation in phase 3 trials. Trial Registration ClinicalTrials.gov Identifier: NCT05454657
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