This randomized, double-blinded, placebo-controlled trial enrolled 146 patients undergoing arthroscopic rotator cuff repair for full-thickness, nontraumatic small to large-sized rotator cuff tears. Patients were randomized 1:1 to receive either a placebo or 500 mg of metformin twice daily, for three months postoperatively. After loss to follow-up, 126 patients completed the study and were included in the final analysis (64 metformin, 62 control). The primary outcome was the incidence of postoperative shoulder stiffness at 3 months; secondary outcomes included American Shoulder and Elbow Surgeons (ASES) score, pain (VAS), patient satisfaction, plasma fibrotic biomarkers, and rotator cuff tendon healing assessed by ultrasonography. Outcomes were assessed at 3 months, 6 months, and 1 year postoperatively. Overall, the results of the study revealed a significantly lower incidence of shoulder stiffness in the metformin group compared with placebo at all postoperative time points, without adverse effects on tendon healing or clinically meaningful differences in long-term functional outcomes.