Abstract
<title>Abstract</title> <p> Background Individuals receiving opioid agonist therapy (OAT) have reduced physical fitness, with negative consequences for health, treatment outcomes, and recovery. This study evaluates the impact of offering a 16-week integrated exercise program on psychological distress and physical fitness in this population. Method This multicentre, randomized controlled trial recruited 311 participants from seven OAT clinics in Bergen and Stavanger, Western Norway. Participants were randomly assigned (1:1) to either an exercise intervention group (n = 156) or a control group receiving standard OAT (n = 155). The intervention consisted of supervised endurance and strength training sessions twice weekly for 16 weeks. Psychological distress was measured using the 10-item Hopkins Symptom Checklist (SCL-10), and physical fitness was assessed using the number of step cycles completed in a 4-minute step test at baseline and post-intervention. Analysis followed an intention-to-treat approach, using generalized linear models to evaluate changes in distress and fitness, presented with coefficients and 95% confidence intervals (CI). Results Adherence to the intervention was low, with only 3% attending ≥ 50% of the offered exercise sessions and a mean attendance of 2.1 <italic>±</italic> 5.0 sessions. No differences were found between the groups in the mean change in SCL-10 score (-0.12,-0.25;0.01) or step cycles in the 4-minute step test (3.68,-2.55;9.91). Conclusion In this real-world effectiveness trial, offering a structured 16-week integrated exercise program resulted in low adherence and did not improve psychological distress or physical fitness. These findings highlight implementation challenges of exercise interventions in OAT settings and suggest that potential benefits depend on achieving sufficient engagement and intervention dose. Trial Registration ClinicalTrials.gov.no NCT05242848. Date of registry: February 16, 2022. </p>