Vitamin D Adequacy Conditions the Prolactin-Suppressive Effect of Metformin in Men Receiving Prolactin-Elevating Medications

This study investigated the role of vitamin D in enhancing the prolactin-lowering effect of metformin in 75 men with antipsychotic-induced hyperprolactinemia and type 2 diabetes or prediabetes. Participants were divided into three groups based on their vitamin D status and supplementation. After six months of metformin treatment, improvements in glycemic control and insulin sensitivity were observed across all groups, with greater effects in those with sufficient vitamin D. Notably, reductions in prolactin levels occurred only in men with adequate vitamin D (groups 1 and 3), suggesting that sufficient vitamin D is necessary for metformin to effectively lower prolactin. Additionally, modest increases in LH and testosterone and improvements in sexual functioning were noted, while vitamin D levels and other hormonal parameters remained unchanged.