Introduction. Rapid weight loss after bariatric surgery is a risk factor for the development of gallstones and its complications. The administration of ursodeoxycholic acid could be a preventive alternative. Aim. To compare the incidence of gallstones in patients treated and not treated with ursodeoxycholic acid and, secondarily, to evaluate the rate of cholecystectomy in each group. Materials and methods. Observational, retrospective cohort study of two non-randomized populations of patients

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... dergoing gastric bypass surgery. The first population was collected retrospectively and the second prospectively and consecutively. Group 1 (without intervention) underwent surgery between January 2016 and December 2018 and did not receive medication, and group 2 (with intervention): operated between May 2019 and October 2020, to whom ursodeoxycholic acid was administered at 600 mg/day for six months. In all cases, abdominal ultrasound was performed at six and twelve months after surgery. Results. In group 1 (n = 73), 21 patients (28.7%) presented an ultrasound finding of gallstones at 6 months, 2 at 12 months (2.73%), for a total of 23 patients (31.5%). In group 2 (n = 32), 4 patients (12.5%) presented a diagnosis of gallstones in the first follow-up period and 1 case in the second, for a total of 5 patients (p = 0.067). Eighteen cholecystectomies were performed due to clinical manifestations related to gallbladder pathology: 17 cases in Group 1 (24.65%) and 1 in Group 2 (3.12%), with a statistically significant difference (p ≤ 0,0435). Conclusion. We have observed a lower incidence of gallstones in the group that received ursodeoxycholic acid, which was clinically relevant, without reaching statistically significant differences. On the other hand, a significant decrease was observed in the number of cholecystectomies one year after bariatric surgery.