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Introduction: Cabergoline is the treatment of choice for prolactinomas. However, 10–20% of prolactinomas are resistant to cabergoline. Metformin, a biguanide widely used in the treatment of diabetes mellitus, has been shown to reduce prolactin secretion in various pituitary tumor-cell lineages both in vitro and in vivo and in human pituitary adenomas in vitro. The aim of this study is to test the effects of metformin addition to cabergoline treatment on prolactin levels in patients with resistant prolactinomas. Subjects and Methods: This is a prospective study performed in an outpatient clinic in a reference center. Ten adult patients (26–61 years) with prolactinomas (7 M), persistent hyperprolactinemia (38–386 ng/mL) under cabergoline treatment (2–7 mg/week) for at least 6 months (6–108 months), features of metabolic syndrome, and not taking metformin were included. Metformin (1.0–2.5 g v.o./day) was given according to patients' tolerance. Cabergoline doses were kept unchanged. Serum prolactin levels were measured before and after short- (30–60 days) and long-term (120–180 days) metformin treatment. Results: Mean prolactin levels did not show any significant changes (148 ± 39 vs. 138 ± 42 vs. 133 ± 39 ng/mL, before, at 30–60 days, and at 120–180 days, respectively, p = 0.196) after metformin (mean dose: 1.25 g/day; range: 1.0–2.0 g/day). No patient reached a normal prolactin level during metformin treatment. Two patients were considered partial responders for exhibiting prolactin decreases ≥50% at a single time point during metformin. Conclusion: Metformin addition to ongoing high-dose cabergoline treatment in patients with cabergoline-resistant prolactinomas failed to show a consistent inhibitory effect in serum prolactin levels. [ABSTRACT FROM AUTHOR]